This fixes:

  Traceback (most recent call last):
    ...
    File "slapos/recipe/build/__init__.py", line 268, in install
      return self._shared.install(self.__install)
    File "slapos/recipe/__init__.py", line 149, in install
      os.makedirs(parent)
    File "/usr/lib/python2.7/os.py", line 157, in makedirs
      mkdir(name, mode)
  OSError: [Errno 2] No such file or directory: ''

It was redundant with conditional configuration sections.
SlapOS fork of zc.buildout has 'multiarch' value since 2.7.1+slapos015

This makes 'init' able to set other options that depend on location.
It's still free to change options['location'] if the part is not shared.

Currently only useful for buildout.hash.cfg files in
https://lab.nexedi.com/nexedi/slapos (e.g. software/slaprunner)

The download recipe relies on buildout's download, which uses hardlinking for
performance reasons in 2 cases: downloading from cache or from local file.

This means 2 things:
- the user shall not modify downloaded files without first making sure
  that st_nlink == 1
- the source file shall not be modified in-place, which is reasable to assume
  for both cache and local file

In-place modification of files is more and more rare because it has many
drawbacks and it tends to be limited to things like logs and databases.
This commit adds a test to check that Git does not do that when managing
working copies: and actually, this may be our only use case of local download.

This commit does not mean that the current way of hardlinking is fully fine.
There remains at least the issue that file permissions may be changed at the
end of the recipe, either with `shared=true` or with `mode` option.

By default, recent SSH (since v8.6) refuses to connect to not-so-old
server (e.g. the version on Debian 8) with RSA key.

It was partially broken since commit 8a9e3766:
it only works when combined with `environment` option.

These files are usually so small that it's free (< FS block size)
and it is more readable with non-str option values (possible with
slapos.buildout).

This fixes a regression that was introduced in
commit d5f5d399.

Traceback (most recent call last):
  ...
  File "slapos/recipe/build/__init__.py", line 191, in pipeCommand
    raise zc.buildout.UserError('Failed while running command:'
  File "slapos/recipe/build/__init__.py", line 192, in <genexpr>
    + ''.join('\n  ' + q for q in failed))
TypeError: cannot concatenate 'str' and 'tuple' objects

With only 384M, debian-11.0.0-amd64-netinst.iso fails to initialize
the network interface.

Since we switched to running tests with python -m unittest discover,
doctests were not longer ran.