The url_for function was defaulting to the public endpoint if no
specific endpoint type was passed as argument.
Also switch from admin to internal keystone endpoint by default and
replace use of a hardcoded endpoint type by the corresponding
configuration option.
As explained in [1], a recent devstack change switched off the creation
of an admin endpoint for keystone. Now that we support configuring
endpoint types, switch to using the public interface for keystone when
deploying blazar with devstack.
[1] https://review.opendev.org/c/openstack/blazar/+/816627
Change-Id: If20c20f6cfb6aa23cb6e19020301bf59044aa79c
34b848aff2