Pin accessories to tags

[dhh]

Allow accessories to specify which hosts they'll run on by using tags.

Example:

servers:
  web:
    hosts:
      - fizzy-app-101: writer
      - fizzy-app-102: reader

accessories:
  beamer-primary:
    <<: *beamer-accessory
    service: beamer-primary
    cmd: beamer primary --remove-after=1h --dir=/storage
    tag: writer

  beamer-replica:
    <<: *beamer-accessory
    service: beamer-replica
    cmd: beamer replica --primary=http://fizzy-app-101:5000/ --dir=/storage
    tag: reader

[dhh]

cc @kevinmcconnell

[dhh]

Added that you can either specify a singular tag or an array of tags, like host and hosts. We should follow up to do the same for roles by adding a singular role.