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
Correct two bugs with neutron router gateway id parsing

1. The regex ended with `\}` but in the case of external gateways such
   as `{"network_id": "8b43562d-f168-4317-8c9f-4d639ba126ea", "enable_snat": true}`
   this was not finding the network_id.
2. The `.*` was greedy and in the prior case, matching more than just
   the id.

Change-Id: I224c0bcf387434bdd029b4c80934a66b60cc77e0
Merge "Improve Neutron subnets management"
Merge "Correct two bugs with neutron router gateway id parsing"