Use debian instead of ubuntu

ansible/roles/prepare_node/defaults/main.yml

-docker_version: "5:18.09.2~3-0~ubuntu-bionic"
+docker_version: "5:18.09.4~3-0~debian-stretch"

ansible/roles/prepare_node/tasks/docker.yml

@@ -14,7 +14,7 @@
 
 - name: Add docker apt repo
   apt_repository:
-    repo: "deb [arch=amd64] https://download.docker.com/linux/ubuntu {{ ansible_facts['lsb']['codename'] }} stable"
+    repo: "deb [arch=amd64] https://download.docker.com/linux/debian {{ ansible_facts['lsb']['codename'] }} stable"
   become: true
 
 - name: Install docker

test/ci-bootstrap.py

@@ -121,7 +121,7 @@ def main():  # pylint: disable=too-many-statements,too-many-branches
         if args.create_droplet:
             # Create droplet
 
-            # image: 18 = Ubuntu xenial 18.04 x64
+            # image: 15 = Debian stretch-x64
             # ssh_keys
             # - 411: ci, ed25519
             # - 407: varac
@@ -141,7 +141,7 @@ def main():  # pylint: disable=too-many-statements,too-many-branches
                 region='ams1',
                 size=4096,
                 disk=8,
-                image=18)
+                image=15)
             droplet_id = droplet['droplet']['id']
             log.info('Created droplet id: %s', droplet_id)
             cosmos.wait_for_state(droplet_id, 'running')