mute useless console output, when run ./stack.sh

Run ./stack.sh will dump ~400 lines of information, because of
tar xvfz pip-*.tar.gz, and python setup.py install.

We'd better mute stdout for the two steps, to make console cleaner

Change-Id: Icf87947e020acb48d8cbe4cdcc1641f060e50f6d
Merge branch 'master' of github.com:openstack-dev/devstack