Added a method to get the domain object from ae objects

Namespace, Class, Instance, Method objects in Automation model now
all have a method called domain which returns the domain object
1. last nil not necessary
2. Seems like a lot of `return`s and `if`s. Hard to read.
3. Adding `domain` here to make like other `miq_ae_*` is very nice.

maybe:

```
def domain
  if domain?
    self
  elsif (ns = ancestors.last) && ns.domain?
    ns
  end
end
```