a number of improvements and fixes to the new mms-agent recipe:

- fix the cookbook name in runit
- adds 2 new attributes: mongodb.mms_agent.{install_dir, log_dir} to
  customize the agent
- forces the use of the standard community runit cookbook
- only restarts the agent service if a change has been detected
- sets two new 'dynamic' attributes for reference:
  mongodb.mms_agent.{version, checksum}

note that the last (bounce on change) required quite a bit of extra work
because it always downloads the zip; most of this can be removed when
the issue is resolved by the mms mongo team

If this changeset is too much, I can always remove the change check, which would greatly simplify this.