Add Environment Helpers installer framework

Add Environment Helpers installer framework

Environment helpers are meta-plugins that allow to set ipsilon in
well defined environments.
For example when ipsilon is install in a FreeIPA or AD domains and
authentication methods, cetificate, keytabs etc, can be pre-configured
and deployed at the same time the server is installed with minimal
effort and wellknown methods.

These are run before any of the other plugins as they can chage the
configuration option for any of the plugins, enable or disable plugins,
or pre-configure some elements.

Signed-off-by: Simo Sorce <simo@redhat.com>