Remove redundant default=None for config options

By default the cfg modules set the default value as None. There
is no reason to set this.

Change-Id: I58e09e01760b573612c2ee169a8dd1784fd36a82
Disallow regular user to update firewall's shared attribute

Shared firewalls should only be operable by  admins.
Currently only admin can provide shared attribute at firewall creation,
so update_firewall should be consistent with that as well.

Change-Id: I093743514637824207b375d724404d51f778d012
Closes-Bug: #1323322
Merge "Remove redundant default=None for config options"