Assign X-Auth-Url header in a separate middleware

authtoken middleware's current purpose is to set X-Auth-Url header to a
value read from heat.conf.

Since this value is not really related to anything the keystone
middleware does and is instead read from a config file, it makes sense
to move it from the authtoken middleware into its own middleware

This change is the first step to grouping all X-Auth-Url related logic
into one single middleware as opposed to have it scattered (or possibly
repeated) in auth_token and auth_password. For example, auth_password
also has some logic around it related to multi-cloud, which can be
extracted and moved to auth_url middleware in a later patch, so that all
handling of X-Auth-Url occurs in one place.

Also, by extracting the X-Auth-Url logic, it allows cloud
providers to remove auth_token or auth_password without side-effects.

Closes-Bug: #1259364
Change-Id: Ieb251c18aa091391a28a90c495b61cf41436f8b9
Merge "Assign X-Auth-Url header in a separate middleware"