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
(cherry picked from commit c8989e62a51b22acf4f7862aa56bf574e9ac19f3)