Add documentation for some API parameters

Add documentation for some parameters which can
be present in glance-api.conf:

* use_user_token
* admin_user
* admin_password
* admin_tenant_name
* auth_url
* auth_region
* auth_strategy

Fix for bug 1256952.

DocImpact

Change-Id: Ia5b731cabaec961f3a4226d30abd3bca9bd87651
Merge "Add documentation for some API parameters"