Don't require pypi.python.org to install pip

[Joe Gordon]

http://git.openstack.org/cgit/openstack-dev/devstack/tree/tools/install_pip.sh#n47

PIP_TAR_URL=https://pypi.python.org/packages/source/p/pip/pip-$INSTALL_PIP_VERSION.tar.g

hard codes the only mirror that we can get a new pip from as pypi.python.org, but as infra has found out depending on external infra such as pypi.python.org leads to failures in the gate. See http://logs.openstack.org/66/56466/1/check/check-swift-devstack-vm-functional/063a31a/console.html#_2013-11-14_21_18_22_913

So we want to be able to make devstack use pypi.openstack.org instead, but don't want to change the default location as we don't want to force people to use our mirrors.

[Jeremy Stanley (fungi)]

Might want to consider this a duplicate of bug 1257079 and fix them both together (PyPI and Github being external requirements for installing pip).