Add gear statsd support

Gear 0.5.0 added statsd support for the gear server.  Have Zuul
use it based on the existence of the same env vars Zuul uses
to configure its own use of statsd.

Change-Id: I09ea2541d1f2eff5f4fd2e0c78da2acfdc1776ab
Retry jobs after gear disconnect

If Zuul is disconnected from gearman, have Zuul retry the job.

Also, report slightly more useful status words for various kinds
of failures when able.

Change-Id: I2d0f2986199516f6251bb122ab619ac56a87ee8e