Add auth_admin_prefix and custom auth_uri params

auth_admin_prefix allows the prepend of a prefix to the URL used for
admin tasks.  See: https://review.openstack.org/#/c/7156/ for
implementation in Keystone.

auth_uri allows to specify the public authentication url (Keystone's
public endpoint) if it can't be guessed from $auth_host + $auth_protocol

Implements blueprint serve-keystone-from-wsgi

Change-Id: Ic66727f78406d70b5ab5bc07fcc074477df2303c
Merge "Add auth_admin_prefix and custom auth_uri params"