Implement CD for launchpad and symphony parsing logic

[benc-uk]

This is another mother of all PRs :( With many changes

• cmd/cd.go handles the run sub command and parses input symphony yaml
  • Able to run against all levels or just single level
  • Each stack inside the level is processed
• symphony/cd.go contains the implementation code for running actions against levels, all levels and stages
• symphony/symphony.go required some changes to expose inner struct types
• MANY changes to the existing v1 symphony format, see #36
• Factored version to it's own package to solve circular import issue
• Simplified code flow in landingzone.go so that Execute is the main function handling all actions
• Adds RunInit: bool to Options to allow actions to skip running the init stage

Fixes #30