This change also replace break by continue, in order to try next computer,
if others computers exist.

The tests runs always 1 test per available computer (no more
no less), this approach prevent pick several tests on the same
machine which leads to timeouts and lack of resources (Disk) on
some machines.

If a software release fail to install because it was not found, there is no
reason to tear down it.

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.)

This reverts commit 7f03294b.

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.