Add a shelve/unshelve scenario

Current test of shelve/unshelve in api/compute/servers/test_server_actions
only checks the API. Nova Bug #1257594 shows that shelve was once not
reliable. This addition should prevent this bug from ever coming back.

Change-Id: Ie9c28e0f17e25f75ce435ac574244cd3c3c69c9a
Merge "Add a shelve/unshelve scenario"