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 SSSD related things need to be hidden behind a specific parameter, which may default to true.  They're not primary to this module, so use of this module + one for SSSD would conflict - users will need the ability to turn SSSD related bits off and manage them elsewhere.