Adding support for Kerberos Ticket based Single Sign-On

Implementation of SSO as per: http://talk.manageiq.org/t/enhancement-to-support-single-sign-on/164

@kbrock if you can kick the tire, that'll be appreciated. keeping as WIP for now.

- Updated Appliance Console to configure Kerberos Authentication for Apache
- Updated Apache server to serve support proxy based pages, needed for Kerberos ErrorDocuments.
- Updated kickstart to copy the proxy based pages
- Updated dashboard controller to support kerberos authentication
- Adding UI Enable Single-Sign On button and configuration for external auth (httpd)
- Support hidden SSO login button to differentiate Kerberos vs. normal IPA login.
- Redirect no credentials normal authenticate to kerberos when SSO is enabled.