Adding support for configuring External Authentication

- New menu entry "Configure External Authentication (httpd)"
- Checks for pre-requisites
- Validates the IPA Server is reachable
- Uninstalls/Re-Installs and configures the IPA Client
- Configures ipa/pam/sssd/httpd/SELinux
- Restarts sssd and httpd
I wonder if we want to use LinuxAdmin gem instead of awesome spawn here.