Make --debug flag on run_tests and run_tempest use testtools.run

This commit changes the --debug flag on run_tests.sh and run_tempest.sh
so that instead of setting set +o xtrace will run tests with testools.run.
This enables pdb to work because calling subunit.run or using testr will
interfere with pdb.

Change-Id: I9953af7a2375a1d26df69559799be3df973756af
Merge "Make --debug flag on run_tests and run_tempest use testtools.run"