Make pypi-mirror more secure and robust

There's no reason we need to run this mirror as root, so run it as a
system user. Also it is creating a cache of removable data, which really
belongs in /var/cache, so we put the data there instead of /opt/stack.

Change-Id: I47d436733e9dd6ca0bebc25bf577e6d749d36f35
Make pypi-mirror use pypi-mirror repo

pypi-mirror was split out from jeepyb.

Change-Id: Ia6c7412d11a6c042ced0ce2780e665e41089f004
Merge "Make pypi-mirror more secure and robust"
Merge "Make pypi-mirror use pypi-mirror repo"