Add donothing function to cmd state.

Feature to make ordering a number of states easier in yaml by grouping them as requisites under a single noop state. This is already possible by using cmd.run name=echo 'something', but wanted this slightly cleaner solution.

```yaml
stage-one:
  cmd.donothing:
    - comment: Database Setup
    - require:
      - postgres_user: main
      - postgres_database: main
      - postgres_group: main
      - cmd: load-db

stage-two:
  - require:
    - cmd: stage-one
```