This allows a list of drop-ins for a unit to be declared inline within a
cloud-config. For example:
  #cloud-config
  coreos:
    units:
      - name: docker.service
        drop-ins:
          - name: 50-insecure-registry.conf
            content: |
              [Service]
              Environment=DOCKER_OPTS='--insecure-registry="10.0.1.0/24"'