Set tenant_id to avoid HTTP 509 error

Hi,

I noticed that  `set_credentials` method do not set `self.tenant_id`. That is a problem because `pyrax` will try to authenticate for each request:

    pyrax/client.py in _api_request at line 207:

    if not all((self.management_url, id_svc.token, id_svc.tenant_id)):
        id_svc.authenticate()

Too many requests in a short amount of time causes Hubic to throw an HTTP 509 error code.

I am not familiar with `tenant_id` concept, so I don't know if it is a good choice to copy `client_id` in `tenant_id`, but it works well.