zuul: configuration for gearman

The next Zuul version we are going to deploy rely on gearman to trigger
jobs. The Jenkins master being a gearman worker.

This change is loosely based on OpenStack Foundation configuration and
brings:
- gearman_server and gearman_server_start parameters
- logging configuration for the gearman server integrated in Zuul and
  for its client side

Change-Id: Ibb146442870c0409988a0728769887dbccef9b3c