Users part of external ldap groups cannot be filtered

1. Configure CB Server with external ldap.
2. Setup group authentication
3. Enable auditing
4. Validate the auditing happens for users that are part of external group
5. Try to add this user as part filtered user list.
Since the user is not present as user in couchbase; filter will not save the changes.

If this is not a bug, then this needs to be release noted too.