Add a Merge::Map feature.

We need to scatter gather in a few situations - determining rabbit
cluster membership, galera membership and configuring hosts for Nova
to permit live migration (which requires host->host communication).
This patch is a proof of concept for an eventual heat feature,
expressed in merge.py. The example given should work for actual use,
but I'll deliver that change separately.

Change-Id: I68e9b2471866810cc698ca3ea28ddf5bb1688d7b