Use explicitely set services urls if no auth_url

Use explicitely set services urls if no auth_url

Refs: #3874

In current configuration, kamaki checks for auth_url. If that fails,
it raises an error. If it is not set, it prints a warning and attempts
to read the <service>.url option from the configuration file.

Service urls: astakos.url, pithos.url, cyclades.url, plankton.url