Add missing create() method on SecurityGroupRule object

The nova.objects.SecurityGroupRule class was missing the
create() method, so we add it here, and while doing so,
ensure that the id attribute is marked as read-only.

Partial-Blueprint: compute-manager-objects-juno
Change-Id: I4584e33d9d163a7dce01b01d205f2f43824e544f
Merge "Add missing create() method on SecurityGroupRule object"