Optionally propagate pillar data from Salt to reclass

Optionally, data from pillars that run before the reclass ``ext_pillar``
(i.e. Salt's builtin ``pillar_roots``, as well as other ``ext_pillar``
modules listed before the ``reclass_adapter``) can be made available to
reclass. Please use this with caution as referencing data from Salt in
the inventory will make it harder or impossible to run |reclass| in
other environments. This feature is therefore turned off by default and
must be explicitly enabled in the Salt master configuration file, like
this:

  reclass: &reclass
      […]
      propagate_pillar_data_to_reclass: True

Signed-off-by: martin f. krafft <madduck@madduck.net>