Disallow unsharing used firewall policy

When admin policy p1 is shared and is used by firewall f1 of different tenant,
then updating p1 with shared=False should not be allowed as it is in use.

Change-Id: I7c753f9d8a25a7edc40233316398475c8ad3efe9
Closes-bug: #1334994
Merge "Disallow unsharing used firewall policy"