Update coredns chart/image to streamline with changes from
https://review.opendev.org/#/c/679525/.
Update osh-selenium image to latest-ubuntu_bionic for python3.
Update prometheus-openstack-exporter to ubuntu_xenial-20191017 for
python3 as well.
Change-Id: I5cefc804e81b718e69b4e8e798444673f7472974