Add quota_usage endpoint for v2 API.

[velp]

This PR adds a new endpoint quota_usage to get all quota usage counters in one API request by project_id. Endpoint URL: /v2/lbaas/quota_usage/{project_id} accept GET request and returns something like:

"quota_usage": {
    "healthmonitor": 1,
    "listener": 1,
    "loadbalancer": 1,
    "pool": 1,
    "member": 2,
    "l7policy": 1,
    "l7rule": 1,
}

For now, this endpoint does several requests to the database to count number of resources, but in the future, we have to use data stored in quota table.

[majewsky]

The quota endpoint uses singular names:

{
  "quota": {
    "load_balancer": 1200,
    "listener": 1200,
    "member": 73,
    "pool": 1200,
    "health_monitor": 200
  }
}

For the sake of consistency, I would prefer the usage endpoint to use the same names. Also of note, later Octavia releases remove the underscores from the names, so it would be loadbalancer and healthmonitor instead, so I would furthermore prefer to use that naming scheme from the get-go here to have consistency with what Octavia will converge upon once we upgrade.

[velp]

For the sake of consistency, I would prefer the usage endpoint to use the same names.

Endpoint v2/lbaas/qouta/{project_id} already taken https://github.com/sapcc/octavia/blob/stable/stein-m3/octavia/api/v2/controllers/quotas.py#L36 so I cannot use it.

Also of note, later Octavia releases remove the underscores from the names, so it would be loadbalancer and healthmonitor instead, so I would furthermore prefer to use that naming scheme from the get-go here to have consistency with what Octavia will converge upon once we upgrade.

Good point! Will fix

[velp]

Never mind, I understood what you mean, will change to singular names

[majewsky]

I cannot comment on the implementation side, but the API side looks good.

[notandy]

lgtm

[majewsky]

We found a bug with this: Deleted loadbalancers get counted into the usage value.

[BenjaminLudwigSAP]

Pushed a fix commit, currently deploying.