Re-enable file-based locking behavior

There are places in Nova (at least) that require file-based locking
behavior for locking on shared filesystems.  These situations can
be recognized because those calls to synchronized specify a
lock_path, so when that happens we need to return a file-based lock
instead of a posix lock.

Change-Id: I21dd17d58d1c321566c72e808a055a0bd625c647