reduce parallelism to improve chance of success

Here are the logs of the bot failures:
In this log the build runs out of loop back devices; potentially because there are too many builds
http://chromeos-botmaster.mtv:8026/builders/x86-zgb-binary/builds/563/steps/Archive/logs/stdio

In this the timeout may be too short for the number of jobs:
http://chromeos-botmaster.mtv:8026/builders/x86-alex-binary/builds/607/steps/Test/logs/stdio

BUG=none
TEST=none
Change-Id: I7e7a7689e121555c8170bb533b8769c40e36ecbd
Reviewed-on: http://gerrit.chromium.org/gerrit/4890
Reviewed-by: David James <davidjames@chromium.org>
Reviewed-by: Puneet Kumar <puneetster@chromium.org>
Tested-by: Puneet Kumar <puneetster@chromium.org>
1 file changed