Standalone servers for keepalive and receiver

 - added supervisor sections for assassind and receiverd
 - keepalive renamed to assasin (name conflicts) and starts as separate process controlled by supervisor
 - RPC receiver moved to separate server

NOTICE: depends on https://review.openstack.org/#/c/72325/

Change-Id: Ib7c74a2b3304cdb5a7c4dbaa57bcbf776b80dcfa
Implements: blueprint nailgun-move-to-uwsgi
Merge "Standalone servers for keepalive and receiver"