Add an option to run_stress.py to run tests serially.

This commit adds a new option to run_stress.py to run the stress
tests serially. If more there is more than one JSON test in the
file passed into run_stress.py it would by default run the tests
in parallel. However, there was no way to specify running the tests
serially. This commit fixes this by adding a -s/--serial option
which will run each test to completion before launching the next
one.

Part of: blueprint stress-tests

Change-Id: I7188929485e29a761327ab24e2bb1213fbd052a9
Merge "Add an option to run_stress.py to run tests serially."