REALM discovery aids

I've added some a cluster_names field to aid clusters, facets, and components as they talk about clusters that have been created within a realm. It's easiest to explain with an example:

```
Ironfan.realm(:p1) do

  # declare control cluster.
  cluster(:control)

  cluster(:es) do
    zabbix_agent{ of cluster_names.control }
  end
end
```

In this case, cluster_names.control will be the symbol :p1_control, representing the name of the control cluster.