CLOUD-2317 (Andy) : Create new pogo plug function to compose and return ...

...a URL to use in order to sign out of HP connected and return to a sign out URL belonging to the caller.  Included spec to validate the URL composition based on the latest URL pattern provided to us by HP.
The `redirect_url` method has a simpler and more Ruby-like implementation for this same behavior.

You can also generalize the logic and make both methods call it.