This reverts commit d41599e5a500163c1010bf02f5cb07bfff14f25c.
Adding Pillow to *requirements.txt was a temporary way to set the
upper bound to < 3.0.0
nodepool thick slaves and requirements have been fixed and devstack is
fixed in the patch that this depends on.
Change-Id: I05bed9356b9840df620c0808a7d0c5c8e3a318c0
Depends-On: I2bec8cf1bfeaaa6ae329704229fdeb86d26e55c7
NOTE(tonyb) Pillow isn't directly needed but it's pulled in via
Collecting Pillow (from blockdiag-> ...)
So cap as per global-requirements until https://launchpad.net/bugs/1501995
is properly fixed
Change-Id: I4b0716569dc8058ccb28cc569f381da5bc30d6a6