Addition of volume/snapshot_metadata CLI

Added below CLIs:-
1. Set or Delete metadata of a snapshot
2. Show metadata of a snapshot
3. Show metadata of a volume
4. Update all metadata of volume
5. Update all metadata of snapshot

Implements blueprint: add-metadata-cli

Change-Id: Ic2b5f3fce6104d1756879718f666a42549458ad3
Merge "Addition of volume/snapshot_metadata CLI"