Add rule for updating network's router:external attribute

Add rule for updating network's router:external attribute

Set admin_only rule for update_network:router:external in policy.json

Also, change the default value of router:external from attr.ATTR_NOT_SPECIFIED
to False, because each time we try to get or update a network the dict with
its attributes is extended by _extend_network_dict_l3 function which adds
router:external=False to the dict if this attribute is not specified.
Thus, if the default value is not specified, router:external is considered
to be updated in any case and the policy rule is applied.

Change-Id: I899d98c7d8c9d9863ac5d8f992b6a2d507ec4482
Closes-Bug: 1338880