test: Disable parallel testing in run_nosetests.py

This adds more complexity and confusion than it is worth.
Also, we now have tests that count on the state of the local image
repository, so don't let tests potentially screw each other up.

run_nosetests.py

@@ -40,7 +40,6 @@ def main():
             '--with-coverage',
             '--cover-inclusive',
             '--cover-package=scuba',
-            '--processes=-1',
             '--detailed-errors',
             '--process-timeout=60',
         ] + otherargs,