This patch restricts the deleting of an provider security group only
to the admin thus preventing the tenant from deleting it.
It also prevents a non admin user from adding or deleting rules from
this group.
NOTE: we are using the following policy.json entry to prevent the
creation of a provider security group by a normal tenant:
"create_security_group:provider": "rule:admin_only"
Change-Id: Ie195225654b0c7cd8cfb715691c5a3bb4c8ee13d
bc26f40491