WFLY-3882 ManagementClient Domain Hosts

Addition of a function at ManagementClient that returns the hosts that are included in the domain.
Always use this idiom for this kind of thing:

```
if (rootNode.hasDefined("host"))
```

Never call ModelNode.get unless you are sure what you are getting exists, or you are willing to create the node. ModelNode provides a fluent API, so one effect of that is get(...) actuall mutates the node you call it on.