Add cleanup of ovz dirs and concurrency workaround

Some of the openvz directories were not getting cleaned up after
and instance was deleted. Now those directories are deleted if they
still exist.

We also noticed an issue where, under load, the driver would run
multiple get_next_id calls before creating the next container. This
made it possible for two containers to share the same CTID which
causes the spawn to fail. This solution uses a directory on the
file system to indicate that a CTID is chosen but not spawned and
also a semaphore in the form of the oslo lockutils.

Change-Id: Ibac11d5dc8b9eaaf9d14dacacc85e600ee342c9a