Set multi valued option manually in ceilometer config

openstack-config uses crudini which doesn't support multi valued
option in current version 0.3, duplicate set will overwrite previous
value instead of creating a new one, so we need to do it manually.

Note, this patch also adds a warning for openstack-config.

Change-Id: I1165c229ad4380de3f5dbc2a8e994e12f446f81f
Closes-Bug: #1269480
backport: havana
(cherry picked from commit b6279c1ac11973a5e0c2723ab2207c12e794525f)