Run local tests at lower concurrency.

Currently, we launch #nproc processes to run tests locally. This patch changes the default
to #proc/2, to not overload the system.

Change-Id: I8bca23eb7462a0c497df93f82a60d85835bedbe9
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2972
Reviewed-by: Ishaan Joshi <ishaan@cloudera.com>
Tested-by: Ishaan Joshi <ishaan@cloudera.com>
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3164
Tested-by: jenkins