fix purge rabbitmq_parameter

`resource[:component_name]` is set on creation (`ensure => present`) and
deletion (`ensure => absent`) of the rabbitmq_parameter type but it is
not set when it's removed from puppet-purge.

On the other hand,  `component_name` is set to absent on creation, is
set on deletion and on purge.

```
             resource[:component_name]          component_name
creation          shovel                           :absent
deletion          shovel                           shovel
purge              nil                             shovel
```

When deleting a parameter, use `component_name`
instead of `resource[:component_name]`

lib/puppet/provider/rabbitmq_parameter/rabbitmqctl.rb

@@ -69,7 +69,7 @@ def set_parameter
     key = resource[:name].rpartition('@').first
 
     if @property_flush[:ensure] == :absent
-      rabbitmqctl('clear_parameter', '-p', vhost, resource[:component_name], key)
+      rabbitmqctl('clear_parameter', '-p', vhost, component_name, key)
     else
       rabbitmqctl('set_parameter', '-p', vhost, resource[:component_name], key, resource[:value].to_json)
     end