Update mailinglist from dev to discuss
Update mailinglist from dev to discuss

openstack-dev was decomissioned this night in https://review.openstack.org/621258
Update openstack-dev to openstack-discuss

Change-Id: Iae6a541f2d16328f98cd9f7009db34ada1cadc75

openstack/puppet-tripleo,openstack/puppet-tripleo,openstack/puppet-tripleo