Port os-networks plugin to v2.1(v3) infrastructure

Ports os-networks extension and adapts it to the v2.1/v3 API
framework. API behaviour is identical.

- unittest code modified to share testing with both v2/v2.1
- Adds expected error decorators for API methods

Partially implements blueprint v2-on-v3-api

Change-Id: I94b7b476cbb02725a396725a379417b64afd58a7
Merge "Port os-networks plugin to v2.1(v3) infrastructure"