Use b.pypi.python.org pypi mirror instead of pypi.

Force easy_install to use b.pypi.python.org instead of pypi.python.org.
Make pip fall back on b.pypi.python.org instead of pypi.python.org.
Pypi proper has been extremely flaky lately. Use and an alternative to
try and avoid this flakiness.

Change-Id: I027f31bc30676b6f8cdc6342173263781251191d
Reviewed-on: https://review.openstack.org/22492
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins