api ec2token: allow auth_uri conf to be set via keystone_authtoken

We already require the keystone_authtoken heat.conf section for the
auth_token middleware, and heat_keystoneclient.py.  So this patch
allows us to reuse the auth_uri specified in that section instead
of requiring a duplicate auth_uri to be specified in [ec2authtoken]

Change-Id: Ic5efafb7743aeddca990a4c560b423293108e1e0
Closes-Bug: #1229674
Merge "api ec2token: allow auth_uri conf to be set via keystone_authtoken"