When accessing the snapshot object in Horizon codebase, we do not have
access to the metadata of the snapshot. This is because the
VolumeSnapshot object does not have metadata as a member. So added
'metadata' to the VolumeSnapshot object. The fix should provide access
to metadata.
Added test data and test cases for snapshot object with metadata.
Closes-Bug: 1542468
Change-Id: I4ae2e7a74d3ddbff8493b42f8f86c7002229cf8c