New exception when auth_url is not specified

Certain scenarios into the neutron client will not specify the
auth_url.  This is typically when a token is specified.  However, when
the token is expired the neutron client will attempt to refresh the
token.  Users of this may not have passed in all of the required
information for this reauthentication to properly occur.

This code fixes an error that occurs in this flow where the auth_url
(which is None) is appended to another string.  This results in a core
Python error.

The update will provide a more targetted error message specifying to
the user that the auth_url needs to be specified.  An associated unit
test is also included to validate this behavior.

Change-Id: I577ce0c009a9a281acdc238d290a22c5e561ff82
Closes-Bug: #1241275
Merge "New exception when auth_url is not specified"