fabfile: add python-pytest build dependency

Better than python-nose : http://pytest.org/ . Make sure it is included
in the build environment for the benefit of packages that need to rely
on it. There are none at the moment but it may replace python-nose in
ceph in the near future.

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #4 from dachary/master

fabfile: add python-pytest build dependency