Switch custom actions to being a single file

[benc-uk]

As a side effect of the work on #109 and #122 it has been decided to change the design of custom actions

Instead of a directory containing multiple YAML files, a single YAML file is used. This simplifies the code significantly, it is also felt this will give an easier/better user experience also.

The file used resides in the $HOME/.rover/ directory and is named actions.yaml.
When rover starts this file is checked, if it doesn't exist a default is created, see pkg/custom/default_actions.yaml Note: The contents of this default file are embedded into the rover binary at build time

Example of actions.yaml
Note there is no longer a need for a name: field, the YAML key is used as the action name

finder:
  executable: "find"
  description: "List all terraform (example custom action)"
  arguments: ["{{SOURCE_DIR}}", "-name", "*.tf"]

lint:
  executable: "tflint"
  description: "A linter for terraform"
  arguments: ["{{SOURCE_DIR}}", "--enable-rule=terraform_comment_syntax"]

All other functionality remains the same

[sebastus]

lgtm

I wonder if it'll make life simpler if there's a "do not change below this line" section that defines the built in terraform actions. that way, all actions are the same and come from the same place.