support goss as a builtin check

[ripienaar]

builtin: goss /etc/choria/gossvars.yaml rest as usual for the nagios check

• [x] watcher https://github.com/choria-io/go-choria/pull/949
• [x] puppet https://github.com/choria-io/puppet-choria/issues/172
• [x] docs https://github.com/choria-io/website/commit/150e47e49a432f0030a9c9f979640b7514be4122
• [x] scout agent to update goss file etc
• [x] agent docs

[ripienaar]

agent additions is barebones but enough for now