Add CLI support for "Load directory service" refresh

As per the documentation, the load directory service can be refreshed:
> when a NOHUP signal is sent to the process

Unfortunately, at least of CentOS 7, Kapacitor does not create a pid file so finding and signaling it is not straightforward from, for instance, Puppet.

A pid file could allow something like:
```
# kill -HUP `cat /var/run/kapacitor.pid`
```

Alternatively, the `kapacitor` CLI could support such an action so the following could be used:
```
# kapacitor refresh
```

Start with the kapacitor CLI command entry point and the load directory service refresh path described in the issue. Check how the process currently handles SIGHUP and how CLI actions reach it. Done means `kapacitor refresh` triggers the same refresh behavior, with tests covering the command and refresh request.

Written by the indexing model from the issue text.