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 branch 'master' of https://github.com/openstack-dev/devstack