Old versions of SSSD do not directly support cross-realm trusts between IPA
and AD. This patch introduces plugins for the ipa-advise tool, which should
help with configuring an old version of SSSD (1.5-1.8) to gain access to
resources in trusted domain.

Since the configuration steps differ depending on whether the platform includes
the authconfig tool, two plugins are needed:

* config-redhat-sssd-before-1-9 - provides configuration for Red Hat based
  systems, as these system include the autconfig utility
* config-generic-sssd-before-1-9 - provides configuration for other platforms

https://fedorahosted.org/freeipa/ticket/3671
https://fedorahosted.org/freeipa/ticket/3672

Provides a pluggable framework for generating configuration
scriptlets and instructions for various machine setups and use
cases.

Creates a new ipa-advise command, available to root user
on the IPA server.

Also provides an example configuration plugin,
config-fedora-authconfig.

https://fedorahosted.org/freeipa/ticket/3670