Let flake8-docstrings use multiprocessing again

flake2.2.3 turned multiprocessing on by default which caused it to stall
completely under Jenkins (bug 70197).  That has been worked around by
turning off multiprocessing with `--jobs=1`.

Pin flake8 to 2.2.5 or later and drop --jobs=1 to resume
multiprocessing.

Bug: 70197
Change-Id: I154f65538061730b44d455ef8e1ca94c90b4d869
Merge "Let flake8-docstrings use multiprocessing again"