As test_https_url proves, the separation of url and https-url is correctly
implemented.

The case where test runner is not enabled got broken in
2a457867

This time, we had test to prevent regression.

/reviewed-on nexedi/slapos!440

So that it renders correctly on https://pypi.org/project/slapos.cookbook

I checked with :
 * `python setup.py checkdocs`
 * `python setup.py check --restructuredtext`

as described on https://stackoverflow.com/questions/16367770/my-rst-readme-is-not-formatted-on-pypi-python-org

/reviewed-on nexedi/slapos!441

we are using trafficserver nowadays, varnish recipe is deprecated

with minimal tests to reproduce the issue that instanciation is broken
when no test runner.

note that we don't put tests in "one big test.py", but from the
beginning we separate so that we can put test for each software type in
test/test_$(software type).py and test for default software type in
test/test_erp5.py

The case where test runner is not enabled got broken in
2a457867

With executing assertEqual(expected, given) the results, especially with
dicts or multiline strings, are more readable in case of failure.

As now the signature is parameterized (in the case of the webrunner),
computing the signature may require not-webrunner-based tools (like
a binary belonging to the SR. Then it can only be done on the
export and the import.

This should fix resiliency tests

/reviewed-on nexedi/slapos!439

So that we can use
nexedi/slapos.core!51

/reviewed-on nexedi/slapos!438

Golang needs a recent enough version of ld (>2.2) and of gcc ( > 5.5 )

This also introduces multiple gcc versions in gcc component, to have finer control over which component uses which version of gcc.

/reviewed-on nexedi/slapos!436

kumofs does not build with gcc 6

When ld version is too old (Debian 7 comes with version 2.2 which is too
old), golang testsuite fail with:

binutils_test.go:237: SourceLine for main: expect [{main /tmp/hello.c 3}]; got [{main  0}]

see also https://github.com/golang/go/issues/26584

We explicitly use gcc 8.2, but what we need is gcc > 5.5 because of
https://github.com/golang/go/issues/24046
When 8.2 (or newer) becomes the default SlapOS gcc, there should be no
problem in dropping the explicit version and using ${gcc:location} here.

This version is compatible with the default firefox (52)

geckodriver is also required

New software release, selenium server, which expose a http server to which clients can connect using [`webdriver.Remote`](https://selenium-python.readthedocs.io/getting-started.html#using-selenium-with-remote-webdriver) and then use standard webdriver API to control the browser.

The server is made of :
 * the "hub" to which webdriver connects
 * a Xvfb server
 * some nodes, for different browsers:
   * Firefox 52
   * Firefox 60
   * Chrome 69
 * an IPv6 https frontend, exposing the admin interface and the hub. Note that python's webdriver binding before 3.14.0 refuse to connect to a hub with a non verified SSL certificate, so for now clients must either use the published `backend-url` (which is IPv4) or use a version of selenium eggs >= 3.14. In the future we'll see how [caucase](https://lab.nexedi.com/nexedi/caucase/) can solve this. Also, this behavior might also change with future version of selenium, because it's wrong that it does not verify SSL certificates.
 * a ssh server, to which you can connect to establish a ssh tunnel with port forward and register more nodes to the hub. This can be used to connect a browser running on your desktop and see the test running, run unsupported browsers, maybe also [appium](http://appium.io/docs/en/advanced-concepts/grid/) to run tests on mobile. This is a kind of "extra feature" that seems working, but I'm not sure we can support it.

To achieve this, we had to :
  * update Xorg components and build gtk with a X11 gdk backend  to be able to run firefox 60. Updating xorg/gdk also fixed a bug in firefox 52, before this, firefox 52 was not able to get the screen size and ` window.screen.width` was 0. Now firefox 52 can get the screen size properly but resizing window does not work.
  * update chrome 69 and fix a few missing `$LD_LIBRARY_PATH` in the wrapper.

Everything seems working, including taking screenshots, uploading files and resizing window - except resizing window does not work on firefox 52.

See the included test suite for some example usage. Basically it's same as what we're doing today, but instead of:

```python
driver = webdriver.Firefox(path_to_firefox)
```

we use:

```python
driver = webdriver.Remote(
   command_executor='http://127.0.0.1:4444/wd/hub', # the url or backend-url parameter published by the instance.
   desired_capabilities={
      'browserName': 'firefox', # or chrome
      'version': '60', # or don't set a version if you want any version (browserName, on the other hand, is required)
   })
```


/reviewed-on nexedi/slapos!420

Change jstestnode arguments to accept the [desired capabilities](https://github.com/SeleniumHQ/selenium/wiki/DesiredCapabilities) as a JSON object, so that we can run jstestnode tests on any selenium server ( the target being !420 ), not only saucelabs.

This MR also introduces some JSON schema for instance parameter.

/reviewed-on !429