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
PR Review Changes for configuring External Authentication.

- Instance variable httpdauth replaced with httpd_auth
- Leveraging LinuxAdmin::Rpm methods instead of invoking rpm commands.
- Leveraging LinuxAdmin::Service methods instead of invoking service and chkconfig commands.
Merge pull request #136 from abellotti/appliance_console_external_auth

Adding support for configuring External Authentication