Remove unnecessary volume creation in test_server_rescue

The setup in test_server_rescue is creating two volumes where really
only one shared volume is necessary, so remove the second volume. This
patch also uses the common base class code to wait for the volume
deletion on tear down and makes the volume name random within the
context of the class name for easier debugging.

This patch moves the volume create/delete into the specific test cases
that need them, uses the common base class code to delete the volume and
wait for it to be gone, and also moves the _delete_volume method from
the v2 compute base test class into it's parent so the v3 compute base
test case can also use it with it's own volumes client.

Partial-Bug: #1254772

Change-Id: Id98f65cc2843a97221222e86c60b3859e15af869
Merge "Remove unnecessary volume creation in test_server_rescue"