Ansible launcher: use a socket for reconfiguration

Signals and multiprocessing don't mix well.  Instead of using
signals for things like stop and reconfiguration, use a socket
that accepts simple commands, and use the zuul-launcher
command to send them.

This implements reconfiguration and stopping.  Other commands
(eg, graceful stop, pause, etc) can be implemented later.

Change-Id: I14b1fdc5e3a20f4b1161dbc14705ad424ad13fbd
3 files changed