REALMS

So far, realms are a way of talking about related clusters. They prepend 
their name to the name of the clusters within them and also allow some 
things (environments, common cloud properties) to be merged into each 
cluster.

For example,

```
Ironfan.realm(:p1) do
  cloud(:ec2).bits 64

  cluster(:control) do
     # etc.
  end
  cluster(:kfk) do
     # etc.
  end
end
```

the realm here would result in p1_control and p1_kfk clusters with a
default cloud(:ec2).bits attribute merged in.

Soon, realms should be usable to set up discovery.

See https://github.com/infochimps/jbro-homebase/blob/master/clusters/p1.rb for an extended example.