OS API should implement network association

Nowadays networks are associated with projects automatically during launch of an instance. The network is chosen rather randomly.
OS API should support association of given network and project.

URI: v1.1/{tenant_id}/os-networks/add
Method: POST
Body: {
    "id": "<network UUID or None>"
}
Response body: empty
Response code: 202

If the given network UUID is None, a random free network will be chosen.

Sample request:
curl -X 'POST' 'http://main-node:8774/v1.1/45d2e66674b647c2bee1d1f18ae64e93/os-networks/add -H 'Content-Type: application/json' -H 'X-Auth-Token: b4be16290db84726bc71d10f0947aa13' -d '{"id": "44b6f3d2-ed5f-4711-b2fa-96e29cf13ac0"}'