Do not force latest version for python-pep8

- Currently fc20 has newer python-pep8 version than other slaves.
  Will downgrade python-pep8 on fc20 untill vdsm and engine will apply
  pep8 fixes.

Change-Id: Ia67700a5fc08cd9d2a8e2f390140a1894641403d
Signed-off-by: Kiril Nesenko <knesenko@redhat.com>