We're doing the same merge twice now (once here in and again in `stub_forge_on`).

What if we did something like:
```ruby
  def stub_forge_on( machine, forge_host=nil )
    forge_host ||= options[:forge_host]
    ....

  def stub_forge( forge_host=nil )
    forge_host ||= options[:forge_host]
    stub_forge_on( default, forge_host )
```

Also, a global `options` accessor isn't defined within this module, nor declared in its API documentation. Would this actually work with something like `BeakerRSpec` that doesn't use our test runner?