Usability: Service user token requested with no auth raises cryptic exception

[Eric Fried]

If [service_user]send_service_user_token is requested, but no auth
information is provided in the [service_user] section, the resulting
error is cryptic [1] and doesn't help the user understand what went
wrong.

Note that today this only happens if the conf section is missing the
auth_type option. But this is a pretty good first indicator that the
admin forgot to populate auth options in general.

[1] http://paste.openstack.org/show/623721/

[Eric Fried (efried)]

https://review.openstack.org/#/c/512329/

Cause I proposed the fix before I opened the bug.

[OpenStack Infra (hudson-openstack)]

Change abandoned by Eric Fried (<email address hidden>) on branch: master
Review: https://review.openstack.org/512329
Reason: Superseded by https://review.openstack.org/521947