system: clean up UnitManager interface
initialize: clean up TestProcessUnits()
config/unit: move Type() and Group() out of config
system: clean up TestPlaceUnit()
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:

  #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"'
Merge pull request #268 from crawford/dropins

drop-in: add support for drop-ins