Install prettytable>=0.7 to satisfy pip 6/PEP 440

Also use sudo -H with pip so that it doesn't create a ~stack/.cache
other things can't write to as the stack user later.

Change-Id: I2134c7d8f58f8b83f33150c9ed86d87f8ccba2f3
Merge branch 'master' of https://github.com/openstack-dev/devstack