Temporarily pre-create ~stack/.cache for pip 6

* tools/create-stack-user.sh: Add a ~stack/.cache directory owned by
the stack user and group. This is intended as a temporary workaround
to be able to add -H to sudo pip calls in grenade and devstack
stable branches, and can then be reverted once those land.

Change-Id: Ie74130beef13a0d9485bb962d86188bb64cce022
Co-Authored-By: Adam Gandelman <adamg@ubuntu.com>
Merge branch 'stable/icehouse' of https://github.com/openstack-dev/devstack into stable/icehouse