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

Signed-off-by: Prasanna Santhanam <tsp@apache.org>
(cherry picked from commit 35ee38385ce0c54b3a16c3270db66d3eb3a56d04)
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>