Reuse erp5.util.taskdistribution for better TaskDistributionTool
integration (adds a dependency, removes 2 entry points): it is possible to
create several test agents with same configuration, each testing a subset
of the entire test suite. Especially useful if/when test agents gets the
extra task of monitoring commits on software releases, and they locally
build & instanciate the SR they test.
Use a medium-lived process rather than a short-lived one, to drop state
loading & saving complexity.
Allow a single test agent to run several concurrent tests.
Cleanup pidfile, even when exceptions occur.
Factorise code used to test each step.
Rework configuration file layout, to better use magic "[DEFAULT]" section.
Take keys & certs from configuration file, allowing for per-test certs.
Remove the need for storing using login & password by reusing x509 auth.

This side-effect was removed in "Pass connection helper as a constructor
parameter, not as class property." commit on slapos.core repository
(hash = c2d5a1eb5d256ef580b1afd34633413adee23e8c).

Closes the gap between command-line arguments and configuration file.
Also, make key_file and cert_file optional, as they are only needed by
slap when master_url uses the "https" scheme.

We maintain time stamps locally.

0.23 was released instead of 0.22.

argparse is available from python2.7, but a lot of python2.7 compilations does
not provide it.

As users expect package to work by default, in case if python used to install
package does not contain argparse, add it to install_requires without requiring
any intervention, because default invocation shall work.

It will allow to minimize common dependency and require ZODB3 module.