fixes #6445 - Configure external authentication via FreeIPA.

With these changes, ```foreman-installer --foreman-enable-ipa=true``` will configure external authentication using FreeIPA per http://theforeman.org/manuals/1.5/index.html#5.7ExternalAuthentication, provided the machine where Foreman is being installer (or reconfigured) is IPA-enrolled and HTTP/foreman-fqdn service exists in the FreeIPA server.
I think we can just let the command fail if it's not there (and neither is the keytab) -- ipa-getkeytab comes from ipa-client package which is kind of expected on IPA-enrolled machine which is our prerequisite -> a7f9f96e248ef2e46bfa5ffb56589e93cdf9be3f