On last midcycle meetup was decided to make snapshots optional feature.
On Manila side was added new boolean share driver capability 'snapshot_support'.
So, disable button 'create snapshot' for shares that have
'snapshot_support' attr equals to 'False'.
Allow creation of a snapshot for shares that do not have such attr for
backward compatibility.
Change-Id: Ibaca8b6cdf9d223f94a83baf7686ad0a0dd9ef8c
Depends-On: Id8e11b591a713654f7237cb1b66aeeaebfbf5871
94b8e4e5ea