Gzip testr result files before archiving.

Gzip testr result files before archiving them to static.o.o as the
subunit log files may be quite large. This will speed jobs up and reduce
the space needed on static.o.o to archive logs.

Change-Id: I3c9570a79c8c348c9b70b7c6a98ba5c207f97825
Reviewed-on: https://review.openstack.org/22636
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins