Reenable chance to login for not existing users

When you use LDAP and you try to login using Signo but the user does not
already exist in Katello database you should not end up in error page.
With this patch you are redirected to login page when you can enter
credentials again. Once you login using katello form you are able to use
Signo in future.
Merge pull request #2270 from ares/bug/ldap_login_not_existing_users

Reenable chance to login for not existing users