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
Add helper functions for getting local disk

Following functions are added in order to be used by validate_bdm
and resource_tracker code.
get_bdm_ephemeral_disk_size: return sum of eph disk size of bdms
get_bdm_swap_list: return a swap disk list of bdms
get_bdm_local_disk_num: return local disk number (include eph and swap)

Change-Id: I215970bea72b502042472aa310fe64ffc4304b25
Merge "Add helper functions for getting local disk"
Merge "Make check_instance_shared_storage() use objects"