Forward user-committee ML to openstack-discuss

The OpenStack User Committee is working on merging with the
Technical Committee, and part of that is making sure discussions all
occur in a common place (including planning the committee merger).
The TC already moved its E-mail discussions to the openstack-discuss
mailing list in late 2018, so moving UC discussions to the same list
is imperative.

Change-Id: I9fd6580a65995be235c0f52e65b37a8491edca95

inventory/service/host_vars/lists.openstack.org.yaml

@@ -9,6 +9,7 @@ exim_aliases:
   openstack-operators: openstack-discuss
   openstack-sigs: openstack-discuss
   openstack-tc: openstack-discuss
+  user-committee: openstack-discuss
   airship-discuss-owner: spam
   ambassadors-owner: spam
   community-owner: spam