qa: set, get, list and remove custom metadata for snapshot

qa: set, get, list and remove custom metadata for snapshot

Following test are added:
1. Set custom metadata for subvolume snapshot.
2. Set custom metadata for subvolume snapshot(Idempotency).
3. Get custom metadata for specified key.
4. Get custom metadata if specified key not exist (Expecting error ENOENT).
5. Get custom metadata if no any key-value is added means section not exist (Expecting error ENOENT).
6. Update value for existing key in custom metadata.
7. List custom metadata of subvolume snapshot.
8. List custom metadata of subvolume snapshot if no any key-value is added (Expect empty json/dictionary)
9. Remove custom metadata for specified key.
10. Remove custom metadata if specified key not exist (Expecting error ENOENT).
11. Remove custom metadata if no any key-value is added means section not exist (Expecting error ENOENT).
12. Remove custom metadata with --force option.
13. Remove custom metadata with --force option if specified key not exist (Expecting command to succeed because of '--force' option)
14. Remove subvolume snapshot and verify whether metadata for snapshot is removed or not

Fixes: https://tracker.ceph.com/issues/55401
Signed-off-by: Nikhilkumar Shelke <nshelke@redhat.com>