./run_tests.sh -N fails strangely without python-mox

if you do not have python-mox installed, and try 'run_tests.sh -N', then it will with a strange error.

$ apt-get --purge remove python-mox
$ ./run_tests.sh -N
ERROR

======================================================================
ERROR: <nose.suite.ContextSuite context=nova.tests>
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib/pymodules/python2.7/nose/suite.py", line 208, in run
    self.setUp()
  File "/usr/lib/pymodules/python2.7/nose/suite.py", line 291, in setUp
    self.setupContext(ancestor)
  File "/usr/lib/pymodules/python2.7/nose/suite.py", line 314, in setupContext
    try_run(context, names)
  File "/usr/lib/pymodules/python2.7/nose/util.py", line 478, in try_run
    return func()
  File "/home/ubuntu/lp-810157/nova/tests/__init__.py", line 73, in setup
    cleandb = os.path.join(FLAGS.state_path, FLAGS.sqlite_clean_db)
  File "/home/ubuntu/lp-810157/nova/flags.py", line 144, in __getattr__
    val = gflags.FlagValues.__getattr__(self, name)
  File "/usr/lib/python2.7/dist-packages/gflags.py", line 935, in __getattr__
    raise AttributeError(name)
AttributeError: sqlite_clean_db

other missing dependencies give sensible ImportErorrs.

python-mox is not a runtime dependency, so its not terribly unreasonable that the user would not have it installed. (it is an ubuntu build-dep, though).