Make check_instance_shared_storage() use objects

This changes the check_instance_shared_storage () method in compute manager
to take an instance object.

The change is mostly cosmetic here to remove one more case of an instance dict
being passed by the rpcapi since the instance parameter isn't actually used on
the server end of this call anyway !

Related to blueprint compute-manager-objects-juno

Change-Id: Id22306916d5d32af978955e02ee95e991c733bab
Merge "Make check_instance_shared_storage() use objects"