Remove support for CoreOS Container Linux (#6576)

tests/cloud_playbooks/create-do.yml

@@ -20,7 +20,6 @@
       - sfo2
       - blr1
     cloud_images:
-      - coreos-beta
       - fedora-24-x64
       - centos-5-x64
       - centos-5-x32
@@ -31,12 +30,10 @@
       - debian-8-x32
       - centos-6-x32
       - centos-6-x64
-      - coreos-stable
       - ubuntu-16-10-x32
       - ubuntu-16-10-x64
       - freebsd-11-0-x64-zfs
       - freebsd-10-3-x64-zfs
-      - coreos-alpha
       - ubuntu-12-04-x32
       - ubuntu-12-04-x64
       - ubuntu-16-04-x64