be opinionated: only use get-pip.py

get-pip.py is now on a CDN, and is the prefered way to get pip.

Remove the default path of using pip tarballs from pypi and use
get-pip.py on from here on.

Closes-Bug: #1326539

Change-Id: I0661f7c6913ba6b3e1d00b30e22740d150bfd060
Merge commit 'refs/changes/83/86083/2' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch