Add alias for user's OSO url to /api/token?for=<alias>

[alexeykazakov]

Currently /api/token?for=<cluster_url> requires loading the cluster URL from user's profile via /api/user to construct a proper request (for example /api/token?for=https://api.starter-us-east-2.openshift.com).

We can add a special alias for the user's cluster URL, something like /api/token?for=openshift. Auth service will look up for the cluster URL where the user is provisioned to and will return the token for that cluster. Plus we can add provider_api_url claim to the result:

{
  "access_token": "asdd123213hsd",
  "scope": "user:full",
  "token_type": "Bearer",
  "username": "johnsmith",
  "provider_api_url": "https://api.starter-us-east-2.openshift.com"
}

We also could return the same payload but without the token for /api/token/status:

200 OK
{
  "username": "johnsmith",
  "provider_api_url": "https://api.starter-us-east-2.openshift.com"
}