Pass minion_id to ext_pillar

Pillar data is always targetted at a minion, so it makes sense to
actually pass the minion ID via the ext_pillar interface, rather than
expecting plugins to obtain it from __opts__.

While this patch also changes all existing plugins, it concedes that
there are going to be plugins outside of the tree, continuing to use the
old interface. Such plugins will trigger a warning written to the master
log. This warning can later be made critical as part of proper
deprecation.

Signed-off-by: martin f. krafft <madduck@madduck.net>