Add skip for cinder extend volume with snapshot

Some drivers [1] are unable to perform the operation of extending
volume that has a snapshot. Add a skip so this test does not get run.

[1] https://docs.openstack.org/cinder/latest/configuration/block-storage/drivers/ibm-storwize-svc-driver.html#extend-volumes

Change-Id: Ie988c9a6be396688b4d78d6a31dfffe8525b97b3
2 files changed