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

Ports os-tenant-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: I340c9b1312a3477c63d28f19df9611c95e67cde6
Merge "Port os-tenant-networks plugin to v2.1(v3) infrastructure"