Derive keystone_ec2_uri from auth_uri

Derive keystone_ec2_uri from auth_uri

ec2token auth_uri must be correctly configured for heat to work.
The /ec2tokens path is not an endpoint in its own right, it is
part of the keystone v2.0 API. The keystone_ec2_uri configuration
parameter was only used for ec2 style credentials validation in
the ec2token middleware.  It might have also propagated all the
way into the user_creds table to be stored for each stack were it
not for this chain of failures (removed in this commit):
- X-Auth-EC2_URL header is set in ec2token, but header X-Auth-EC2-Url
  is read by RequestContext
- RequestContext stores the ec2 uri in aws_auth_uri, but UserCreds
  expects aws_auth_url

Change-Id: I9908e17bed33fdc64d058a6d6db7b29f9c8d53d6