Use requirements project list for mirror selection

The openstack/requirements repo maintains a list of projects that have
opted in to receiving automatic updates. Those projects clearly want to
be beholden to the mirror as well, so consume that for mirror choice
instead of trying to figure it out via rules based on name.

Change-Id: I5c65287fabe43fc3e6dec315b29c13788bd3c1ea
Merge "Use requirements project list for mirror selection"