Complete restructure - mother of all PRs

[benc-uk]

Complete restructure and refactor. this is a monster PR

• The command structure now implements the proposal in in the wiki. This means that action verbs are top level commands
• the ci, cd, landingzone and launchpad commands are gone
• Custom commands now replace ci and ci_tasks directory is replaced with custom_actions
• Custom actions can be any external executable, and they are dynamically loaded and extend the rover CLI a bit like plugins, e.g. See the custom_actions folder for two examples rover linter and rover finder
• The operational mode is determined by the presence of either the --config-dir or the --config-file flags, which changes between adhoc or single level mode (without a symphony YAML) and multi-level mode (with a symphony YAML)
• All actions are now implemented as via an interface Action with TerraformAction and CustomAction being implemenatons, and each of the terraform actions are separate implementations e.g. PlanAction and ApplyAction.
• Actions must implement the Execute(o *Options) which is how they are run.

[sebastus]

Can we further isolate config and execute? Something like this:

pkg/landingzone/cobra.go:

func RunFromCLI(cmd *cobra.Command, action Action) []Options {  
...  
   return []Options{opt}  
}

pkg/symphony/cobra.go

func RunFromConfig(cmd *cobra.Command, action landingzone.Action) []Options {  
...  
    if levelName == "" {  
        return conf.runAll(action, dryRun)  <-- returns array of Options  
    }  
...  
    return conf.RunLevel(*level, action, dryRun)  <-- returns array of Options  

root.go

var options []Options  
if configFile != "" {  
    options = symphony.RunFromConfig(cmd)  
} else {  
    options = landingzone.RunFromCLI(cmd)  
}  

for _, optionSet := range options {  
    _ = action.Execute(optionSet)  
}  
console.Success("Rover has finished")  
os.Exit(0)  

Then, a test can do the initial bit to build the options array and assert on its contents without running execute. An integration test can go on to run Execute and check the actual resources.