Fix interprocess locks when running unit-tests

When running unit-tests concurrently using testr, we might
need to use interprocess file locks (e.g. for running various
tests using the same DB in MySQL/PostgreSQL).

At the same time file locks are only usable when they are placed
to one dir, and we have a test fixture that overrides the lock_path
config value for each test case with a new temporary directory.

The solution is to create the one temprorary directory for locks
to be used by all tests before running tests and then remove it
after running tests. The corresponding helper has been already
put to openstack.common.lockutils module, we just need to reuse
it properly, i.e. change the way unit tests are run in tox.ini.

Closes-Bug: #1261728

Change-Id: I76f95a9f7fdd31c15e6cf4fd6316c7569284f780
Merge "Fix interprocess locks when running unit-tests"