Disabled parallel test execution for reviewbot.
Disabled parallel test execution for reviewbot.

While we globally disabled parallel test execution for reviewbot jobs
in `37532fae01d`, we missed that the configuration was overwritten
downstream.

This patch disables parallel tests also in the script ultimately
performing the final configuration.

Review: https://reviews.apache.org/r/69860

gsantovena/mesos,kaysoky/mesos,kaysoky/mesos,jpeach/mesos,chhsia0/mesos,Gilbert88/mesos,dforsyth/mesos,chhsia0/mesos,jpeach/mesos,chhsia0/mesos,gsantovena/mesos,kaysoky/mesos,abudnik/mesos,jpeach/mesos,gsantovena/mesos,reneploetz/mesos,abudnik/mesos,chhsia0/mesos,abudnik/mesos,kaysoky/mesos,dforsyth/mesos,reneploetz/mesos,kaysoky/mesos,chhsia0/mesos,kaysoky/mesos,Gilbert88/mesos,abudnik/mesos,gsantovena/mesos,dforsyth/mesos,Gilbert88/mesos,reneploetz/mesos,dforsyth/mesos,jpeach/mesos,jpeach/mesos,gsantovena/mesos,jpeach/mesos,reneploetz/mesos,abudnik/mesos,abudnik/mesos,dforsyth/mesos,Gilbert88/mesos,abudnik/mesos,chhsia0/mesos,dforsyth/mesos,jpeach/mesos,reneploetz/mesos,Gilbert88/mesos,reneploetz/mesos,gsantovena/mesos,Gilbert88/mesos,kaysoky/mesos,gsantovena/mesos,dforsyth/mesos,chhsia0/mesos,reneploetz/mesos,Gilbert88/mesos