Add support for release network_metadata serialziation

Patch adds support for release network metadata serialization,
by adding additional handler on release/<release_id>/networks for PUT/GET
requestss
Proper validation for release networking data

Why not to add another field in release handler:
1. networks_metadata it is ours internal naming
2. url release/<release_id>/networks makes more sense for client
  applications

Change-Id: I205ec786f1a9d772905ff22e426c1ce8a6d4d6e7
Closes-Bug: 1381557
Merge "Add support for release network_metadata serialziation"