Skip incompatible test on OSX

Can't seem to get the test:
cinder.tests.test_volume.VolumeTestCase.test_delete_no_dev_fails
to run right, tried macports and homebrew versions of fileutils
but still get makedir failures. Give up and just detect if OSX
and skip.

Change-Id: Ic2f7603cf2960e2fbaac9fce677461f60ded541d
Merge "Skip incompatible test on OSX"