Currently we expect keystoneauth to raise on missing endpoint, but in
many cases it returns None from get_endpoint instead. This causes two
kinds of problems:
* Raising Python exceptions in the code that is not ready to handle None
(e.g. get_ironic_api_url in deploy_utils).
* Passing endpoint_override=None to client constructors, thus effectively
ignoring provided adapter configuration options.
This change turns None into a proper error.
Change-Id: I8dad2e8ef5ff39167995dfe8266a48e80a29788b
b2ee05a0b1