Container Auditor should log a warning if the devices path contains a non-directory.

If the devices path configured in container-server.conf contains a file
then an uncaught exception is seen in the logs. For example if file foo exists as such
/srv/1/node/foo then when the container-auditor runs, the exception that foo/containers is
not a directory is seen in the logs

This patch was essentially clayg and can be found in the bug

I tested it and wanted to get a feel of the openstack workflow so going through the
commit process

I have added a unit test as well as cleaned up and improved the unit test coverage
for this module.
- unit test for above fix is added
- unit test to verify exceptions that are raised in the module
- unit test to verify the logger's behavior
- unit test to verify mount_check behavior

Change-Id: I903b2b1e11646404cfb0551ee582a514d008c844
Closes-Bug: #1317257
Merge "Container Auditor should log a warning if the devices path contains a non-directory."