allow create credential with the system admin token

We are looking up trust_id during create credential, which means caller must
use a Keystone-issued token. This is unrealistic as create credential are
often done as part of bootstrap, using the static system admin token.
Furthermore, deployments which using external authorization will break as it
may not have a token_id in the request context.

For the above reasons, we'll skip trust_id lookup if the request token_id is
either absent or it is the static system admin token.

closes bug 1275145
closes bug 1263804

Change-Id: I6cda3c5f36c9754ab84e28ff9a9289887d6c9b77
Merge "allow create credential with the system admin token"