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: I7cd33f95b7baa22dbd4abb6e391ee59876998f59
Merge "Update references with new Mailing List location"