WMF: less build description updates

Whenever a job status change (in progress, completed), Zuul would update
the job description of all other jobs. Each update takes roughly 500ms
according to Jenkins access log.

A change made to Mediawiki core triggers 9 jobs, each of them will have
two states changes hence 18 updates are submitted or 9 seconds (at
least).

This change disable the description update when a build starts or
completes.  The description would only be updated when the change is
reported back in Gerrit.

bug: 48025
Change-Id: I3bf4096659b071fec25370397ffb21dd6ee77426