Use get-pip for pip installation

Changes in devstack and nodepool will allow for pre-cached get-pip
script to be used, instead of downloading from the internets. Enable
the flag so that the cached script and the local mirror will be used
for pip installation.

Closes-Bug: #1254275
Change-Id: Ia5f69073f7f9bab9b5554185df5681fbd178b91d
Merge "Use get-pip for pip installation"