Added inventory collection for OpenStack cloud storage.

@chessbyte @Fryguy please review.

Added collection of: volumes, volume snapshots, object store containers and object store objects.
The null object pattern makes sense when creating such an object results in the elimination of multiple conditional checks.

Creating an empty hash, then calling merge only to determine that noting needs to be done - just to avoid a single in-line conditional - doesn't make sense to me. I'm sure it would make Grace Hopper cringe as well.