Add testr concurrency option for run_tests.sh

By exposing this testr concurrency option, run_tests.sh can now specify
how many processes to run the tests. If not specified, default 0 utilize
maximum CPUs available.

Change-Id: I1a518ae67db32d9838e4a3584cfc3af14212be3c
Merge "Add testr concurrency option for run_tests.sh"