This updates the gerrit upgrade testing job to upgrade from 3.6 to 3.7.
This upgrade requires an offline reindex which is new for us since we've
been on Gerrit 3.x. In order to support this offline reindex requirement
the gerrit role is modified to trigger an offline reindex in the role's
start tasks if the flag to do so is set. I expect this will really only
be used in testing, but it allows us to reuse most everything else in
testing and in production which is nice.
Change-Id: Ibe68176970394cbe71c3126ff3fe7a1b0601b09a
90440837a7