Fixes #2502 - session expiration updates

Signo now allows to transparently prolong user session which got
expired. Also when you now logout from Foreman you will end up in Signo
login form. This brings some code clean up and test changes and small
SSO method API change.

THIS PR MUST BE MERGED AT THE SAME TIME AS:
* https://github.com/Katello/signo/pull/2
* https://github.com/Katello/katello/pull/2310

Please let me know before you merge this.