automatic parallelism does not work on OS X

[Joe Gordon]

When running 'testr run --parallel' on a Intel Core i7 CPU (4 cores) OS X 10.7.5, only one thread is used.

Discovered when running nova unit tests with 'tox -epy27'

[Robert Collins (lifeless)]

We need a platform specific override for this:
testrepository/testcommand.py
    def local_concurrency(self):
        if sys.platform == 'linux2':
            concurrency = None
            for line in file('/proc/cpuinfo', 'rb'):
                if line.startswith('processor'):
                    concurrency = int(line[line.find(':')+1:]) + 1
            return concurrency
        # No concurrency logic known.
        return None

We might be able to query multiprocessing, or you could implement an equivalent codepath for mac.