Adding support for Kerberos Ticket based Single Sign-On

- 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.
PR updates for Kerberos Ticket based Single Sign-On

- Rspec fixes
- Simplifying vmdb config value checks
- Rubocop fixes
Merge pull request #735 from abellotti/single_signon

Adding support for Kerberos Ticket based Single Sign-On