Fix locking in migration tests

The migration test code included an external lock with a hard-coded
relative path that breaks when run in Jenkins.  It wasn't being hit
in Oslo because it was only locking for the postgres code path,
which isn't currently tested in Oslo.

This change moves the lock so that it applies to all db types and
removes the relative path.  For the locks to work properly it was
also necessary to add the lockutils wrapper to the Oslo tox.ini.

Closes-Bug: 1255608
Change-Id: I333b833022f208e48e3b7e82e8cfad2764be5ea0
Drop dependency on processutils from oslo db code

We should remove processutils module usage from common db code
before we split oslo.db from oslo-incubator into a separate library
because common processutils module uses common log module (so,
registers a few oslo.config options) and eventlet.

We use processutils module only as commands module replacement, so
we can use subprocess module from python standard library instead.

Related to blueprint oslo-db-lib

Change-Id: Id5ff507c9da329fa6ac58e54f8f4d0fb574ab04f
Merge "Fix locking in migration tests"
Merge "Drop dependency on processutils from oslo db code"