CLOUDSTACK-4452 - Fixed method is_snapshot_on_nfs and moved it to utils

Signed-off-by: Prasanna Santhanam <tsp@apache.org>
CLOUDSTACK-4452: Cleaning up the is_snapshot_on_nfs utility

- Pass in config and determine the mgmtsvr credentials
- remove the dir_paths param which is not reqd.
- pass all entities as uuids and convert to ids within the method

Signed-off-by: Prasanna Santhanam <tsp@apache.org>
(cherry picked from commit 02e7630f1cf218b1436a71a42623a7bf790d98e7)