Readd run_tests.sh --debug option.

Testr doesn't directly support running tests with pdb so the --debug
option was pulled from run_tests.sh during the testr switch. Add the
flag back into run_tests.sh and have it run tests with testtools.run
which allows users to import pdb in tests then run them without
interference from testr.

Example usage:
  # First edit test file to import pdb.
  run_tests.sh -d path.to.test

If the test is not given on the command line all tests will be run.

Change-Id: I4afca9ca997ccb6c575b714b778a9db8306c7891
Merge "Readd run_tests.sh --debug option."