tox: Use a range of ports for live servers

tox: Use a range of ports for live servers

From:

    https://docs.djangoproject.com/en/1.8/topics/testing/tools/#liveservertestcase

In the case where the tests are run by multiple processes in parallel
(for example, in the context of several simultaneous continuous
integration builds), the processes will compete for the same address, and
therefore your tests might randomly fail with an "Address already in use"
error. For example:

    Traceback (most recent call last):
      File
      ".../python2.7/site-packages/django/test/testcases.py",
      line 1193, in setUpClass
          raise cls.server_thread.error
          error: [Errno 98] Address already in use

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Acked-by: Stephen Finucane <stephen.finucane@intel.com>