Do not install pip when OFFLINE=True

install_pip.sh trys to fetch pip from the internet even if
OFFLINE=True. It causes stack.sh to fail if the environment
is actually disconnected from the internet. With this patch,
stack.sh skips install_pip.sh if OFFLINE=True.

Change-Id: Ica9e5cfa0a4ee684c05393896c2fd6ddbd9ccd06
Closes-Bug: 1240956
Merge "Do not install pip when OFFLINE=True"