Add support for Gemfile.local
Spec tests for the YAML loader

Currently covers the behavior of the `yamldir` method. The `yamlfile` method
is pretty trivial.
Use deep_merge when loading configs from YAML

Adds a dependency on the `deep_merge` gem and uses it to merge configs loaded
by `yamldir`. This allows top-level entries such as `vms` or `roles` to be
broken out into multiple files and allows config in one subdirectory to
override or extend the config loaded from another subdirectory.