Change I8da7dd95ae24cf06dc7bdc300fcf39947a6df093 added Pillow build deps
to nodepool thick slaves. This means that Pillow 3 will work in unit
tests.
Make the matching change to allow Pillow 3.0.0 to work under devstack.
The longer term aim is to remove temporary upper cap.
Change-Id: I2bec8cf1bfeaaa6ae329704229fdeb86d26e55c7