remove read-only network ID field from Edit Network form

Remove unnecessary field network_id from UpdateNetwork form.
Currently field is required to pass it to the
api.neutron.network_update method, but there is also
self.initial dict where the value can be get.

Closes-Bug: 1372348
Change-Id: I5cf9efa468d3f0a57d20e5ae54f74425d01ac606
Merge "remove read-only network ID field from Edit Network form"