Improve Neutron subnets management

Previously, subnet had some parameters as read-only which blocks some
features, like specifing dns servers, allocation pools and also specific
host routes. This patch aims to add support for these options as well for creation
and also for update (for some of them where API allows it).

Change-Id: Ic0e4ccbcc58a92bd014f0095de24e694dc7be6a9
(cherry picked from commit 340e1bb03787835637fe2c0bea86c1e0e6181221)