Add a test for creation of volumes' snapshots.

A new test called 'test_snapshot_create_get_delete' is added.
Also adds setUpClass and tearDownClass methods to create a
shareable volume origin.
Finally updates the existing 'test_volume_from_snapshot' which
now uses the create_* and clear_* wrappers from base class and
the shareable volume origin from setUpClass method.

Change-Id: I3fb8edabaf4649736a43d6de5a9c4518d3caac62
Implements: blueprint add-snapshot-tests
Merge "Add a test for creation of volumes' snapshots."