drop-in: add support for drop-ins

This allows a list of drop-ins for a unit to be declared inline within a cloud-config. For example:

```yaml
#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"'
```