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

devstack-vm-gate.sh

@@ -116,6 +116,7 @@ VIRT_DRIVER=$DEVSTACK_GATE_VIRT_DRIVER
 SWIFT_REPLICAS=1
 LOG_COLOR=False
 PIP_USE_MIRRORS=False
+USE_GET_PIP=1
 # Don't reset the requirements.txt files after g-r updates
 UNDO_REQUIREMENTS=False
 CINDER_PERIODIC_INTERVAL=10