Since a process might exit by himself at any time, handle exceptions when
we try kill processes

This prepares for the removal of --firefox_bin and --xvfb_bin options. The
API between the testnode and the tested software must be exclusively generic.

Conflicts:
	product/ERP5Type/tests/ERP5TypeFunctionalTestCase.py

/reviewed-on nexedi/erp5!505

/reviewed-on !346

This caused two problems:
 - If Xfvb was killed and left a stale lock file, we don't reuse this
   display. After some time we end up having "All display locked"
   exception but no Xvfb process running.
 - Xvfb's slapos is patched to store control sockets and lockfiles in
   $TMPDIR ( slapos@58462c24 ).
   We don't have $TMPDIR set in testnodes, but this would be a problem
   if we ever set it.

Because Xvfb can detect a stale lock file upon startup and start anyway,
the easy way running Xvfb for each display and see if it's still running
after a few seconds.

/reviewed-on nexedi/erp5!269

Fix error display after save
Fix parameters not recognized
Fix python linting
the same way as ERP5 Script Python

Models with workflow state "default" are only considered in composition if no
other valid model is defined. This allows to implement "ingest anything"
policy in wendelin.