Adds Varnishadm Action

Adds the Varnishadm Action.

```
<brood>
    <action class="Brood\Action\Admin\Varnishadm">
      <hostgroup>oyster_varnish</hostgroup>
      <hostgroup>widget_varnish</hostgroup>
      <parameters>
	<host>localhost:6082</host>
	<secret>secret</secret>
	<backend>set_health</backend>
	<matches>
	  <match>backend_1</match>
      <match>backend_2</match>
	</matches>
	<state>sick</state>
      </parameters>
    </action>
</brood>
```