Re-use common volume create/attach methods in test_iscsi_volume

This updates the test_iscsi_volume test to re-use the common
methods from the BaseV2ComputeTest class for creating a volume
and attaching it to a server, along with common cleanup handling.

Change-Id: I2ca16a521251e7af0ec9fb6f1ad92ce44d485437
2 files changed