Add tools/install_pip.sh

This extracts the useful bits of installing pip and friends from source
to get known working versions.  Once the script itself works we'll point stack.sh to it.
Tested on Precise, F18 and CentOS6.

Known improvements:
* cache the downloaded scripts in devstack/files so the case cache can pre-load them
* improve the pip version checking

Change-Id: Ibb4b42119dc2e51577c77bbbbffb110863e5324d
Merge branch 'install-pip' into dt-test
Merge branch 'screen-stop' into dt-test
Merge branch 'doc-shocco-cleanup' into dt-test
Merge branch 'update-usb-tools' into dt-test