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

Change-Id: I2134c7d8f58f8b83f33150c9ed86d87f8ccba2f3
(cherry picked from commit 1a89f8cdf4791350a9aacb41037ab3bdcc9ecc9f)
Merge branch 'stable/icehouse' of https://github.com/openstack-dev/devstack into stable/icehouse