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?
Add a swap-out graphite-data check

Checking for actual paging is a better check than monitoring the
amount of swap used -- primarily in that it will alert when paging
occurs.

When the paging stops the alert will clear, so it's important to
check this over a reasonable period of time - in this instance the
`--from -30mins` should ensure that we are in an error state for at
least 30 mins.

I've set the defaults to be '> 0' for warning, '> 10' for critical,
mostly because we will want to be aware of any paging activity. We
may want to adjust this to be less sensitive later.
Adjust swap defaults

Now we have swap-out monitoring, we can ease off the swap-free thresholds