Remove X-Auth-Url header from authtoken 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 the context middleware

With this change, it makes it possible to authenticate against keystone
prior to reaching heat, forward the auth headers, and still have all the
necessary information on the context.

Closes-Bug: #1259364
Change-Id: Ieb251c18aa091391a28a90c495b61cf41436f8b9