Update references with new Mailing List location

Yesterday, openstack@lists.launchpad.org was migrated with
all users to openstack@list.openstack.org.

This patch updates references to the old mailing list with the
new, to ensure that people encountering them don't accidentally
try and join the old list!

Change-Id: I76242f4f4f0c8c2418068ea709ab2f7aa4502fd5
Merge "Update references with new Mailing List location"