Network can be quite unreliable, so play safe retrying the method in case
some exception related to network failure be raised. Usually they are
related to timeout on SSLError and/or xmlrpclib timeouts.

Make test agent delay more time for 2 reasons:

  - slapgrid-cp/sr takes more them 1 min to run
  - The default periodicity on the servers are 5 min, so nothing happens in
        less them 5 minutes.

This change also gives time for vifib master process the information with
activities.

This changes improve the log readability in order to identify when a test
start and stops (tears down.)

Conflicts:
	CHANGES.txt

Conflicts:
	CHANGES.txt
	setup.py

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.