Export and Import from a single YAML file

@Fryguy @gmcculloug @chessbyte @tinaafitz 
For export and import of automate models we have 3 ways to store the data based on YAML. We currently have (1) ZIP file that can store all the YAML's or (2) store all the YAML's in a directory. This PR adds a 3rd one which is to have all the model into a single consolidated YAML. The single consolidated YAML helps us with our specs where we don't have to store all the model YAML files separately in a directory.
Don't use the `\` line continuation.  In this case, just do

```ruby
unless File.exist?(@options['yaml_file'])
  raise MiqAeException::FileNotFound, "import file: #{@options['yaml_file']} not found"
end
```