Allow more http load to gerrit

Around UTC evening each day gerrit is getting unresponsive. Since
ytterbium is mostly idling, we allow more http traffic to the host. It
will probably not fix the UTC-evening-unresponsiveness, but it could
help during very busy times or against not too crazy bots.

Default of httpd.minThreads was 5.
Default of httpd.maxThreads was 25.

Change-Id: I3d68006157abdb82fefd0aaed9d380cdff398d4a