Support use of token value in place of password.

blueprint horizon-webui

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While python-novaclient will re-authenticate using an internally
saved copy of an authorization token, this capability is not
exposed to its API user requiring the use of a password whenever
creating a Client object.  In order to authenticate requests made
on behalf of a web (horizon) user this capability needs to be
utilized as the horizon web app has only a token once the user
logs in.  This change depends on the structure of the client
and is the first place to look for problems once the underlying
API software is revised.

Partially implements: blueprint horizon-webui

Change-Id: Ifa9cb7877eabff1514cb2272407d6daf08beef7f