Revert "Add pypi-openstack and pip-cache to common elements"

This reverts commit 54e7c72afae9f984c4ca8d4b57bba79a4eebbc25

pip-cache is not concurrent build safe, and the pypi mirror isn't available
over https, nor is it CDN'd.

Change-Id: I19e2a8781bb1d9053ebaf33828a7658295abf486