Workaround flake8 2.2.3 breaking with pep257

pep257 yields some error which apparently cause flake8 2.2.3 to stall
completely waiting for some input.  That can be worked around by
disabling multiprocessing (turned on by default with 2.2.3). Simply pass
--jobs=1 to flake8 in the flake8-docstrings job.

Upstream issue:
https://bitbucket.org/tarek/flake8/issue/167/flake8-223-stall-with-multiprocessing

Change-Id: Iff84c926b42846b4c36db30507851859a827e103