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

Ports os-networks-associate 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: I6ccf9613abf999784c548fb058522ac60db7f052
Merge "Port os-networks-associate plugin to v2.1(v3) infrastructure"