Add new snapshot compute feature flag

Nova image creation / instance snapshotting is not supported across compute
drivers.   This adds a new compute feature flag and skips tests that attempt
to create snapshots.

Change-Id: I814157d434e3d5e13dfae9b9ee7d5135199c4e97
Merge "Add new snapshot compute feature flag"
Merge branch 'master' of https://github.com/openstack/tempest