[APPS-924] Use default settings if present
Refactoring to extract settings_for_parameters into its own module
Make settings a class which uses get_value_from_stdin from the interface passed in
Refactor specs for readability
Rename settings_for_parameters to get_settings_from, receiving user_input object rather than interface property
Merge pull request #24 from zendesk/daniel/APPS-924

[APPS-924] As an App developer, I expect ZAT to take my default settings into account