Plugin update requires a client restart

At least when using EMBEDDED_RUBY=true, it seems that we need to
restart the client to pick up updates to the plugin/*.rb files.

Noticed this with the recent update to ruby-graphite.rb
sensu-server needs to be restarted if checks added

Sensu server needs to be made aware that there are new checks to
distribute.

Noticed this during addition of the swap-out graphite check.
Fix corner case where no roles are defined.
Bug with watching recursed directory?

Making this change as we got:

```
The following requisites were not found:
  watch:
    file:
      /etc/sensu/plugins/*
```

... probably because the resources are not named directly?
Simpler default fix using grain.get
Merge pull request #33 from ministryofjustice/fix_watchers

Fix watchers