ec2token tolerate fully-qualified ec2authtoken auth_uri

If (as is currently done by the puppet-heat manifiests) the ec2authtoken
auth_uri is specified, and it looks like http://127.0.0.1:5000/v2.0/ec2tokens
ec2token authentication will fail, because we always append "ec2tokens" to
the auth_uri.  Instead, only add it when needed.

This probably needs to be combined with a change to the puppet manifiests so
we don't set the ec2authtoken auth_uri, as it's an optional section - we can
derive the correct path with only the auth_uri from the keystone_authtoken
section.

Change-Id: I8c89772b40523b30f1c46b2ca8b68f9e20d5c213
Closes-Bug: #1318599
Merge "ec2token tolerate fully-qualified ec2authtoken auth_uri"