Use Zuul's internal gearman server.

(This is only used by zuul-dev currently.)

Remove co-hosted gearmand server (will require manual cleanup).

Also, listen for feature/gearman events in openstack-infra.

Change-Id: I2dd58e62ff79277f1ef5c79da54bcc7511f2ccee
Reviewed-on: https://review.openstack.org/30175
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins