Skip volume snapshot tests if feature is not enabled

Not all cinder volume backends support volume snapshot. For instance,
the default NFS backend doesn't. This patch adds a new entry to
CONF.volume_feature_enabled to add the possibility to skip volume
snapshot tests.

Change-Id: I40ce17924bbe4eb6f1c941df974bfa832ec2a999
Merge "Skip volume snapshot tests if feature is not enabled"