Add support for builtin -hosts and -null maps

[exptom]

I thought I would have a go at adding the feature that I needed. (see #12 )

This allows you to utilise the builtin maps (-hosts and -null) provided by autofs. e.g.:

autofs_maps:
  - name: home-nfs
    mountpoint: /nfs
    options:
      - "-hosts"
      - browse
      - "--timeout=300"
      - "--negative-timeout=30"

The role now checks if -hosts or -null is included in the options array. If it is then it does not create the map file and you do not need to define directories.

I'm unfamiliar with molecule and encountered some errors getting it running - I'd appreciate pointers for adding suitable tests to this PR.

Thanks!