if we have a valid auth token, use it instead of generating a new one

If authenticating with a valid token (i.e., not username/password
auth), novaclient would instruct keystone to issue a new token with
an identical expiry to the one used, and pivot onto it.  Since
there's zero benefit to doing that, make it keep using the one it
has after validation

Fixes Jira: OPENSTACK-479

Change-Id: Ic4f0e6d506d8d81f0b1d46138e7e19d3b3392e1d
Merge "if we have a valid auth token, use it instead of generating a new one" into metacloud/grizzly