[WIP]-Delete a specific gear/gears

Usage -
1. Pass more than one or more gear_uuids for deletion by mentioning them in the gear_uuids parameter as follows-
curl -k -X POST https://broker.example.com/broker/rest/application/<app_uuid>/events --user username@example.com --data-urlencode event=scale-down --data-urlencode gear_uuids=gear_uuid1,gear_uuid2
We need to perform validation on the user-specified gear_uuids. An exception should be raised if any of the gears do not exist in the application. The gear_uuids should also all belong to the group instance that is being scaled down.