Create config migration commands for run and inject

[florisvdg]

New commands:

Migrate config template files

secrethub migrate config templates <path/to/tpl>

Migrate secrethub.env files

secrethub migrate config envfile <path/to/secrethub.env>

Migrate secrethub:// references in configuration code

secrethub migrate config references <path/to/file>

Examples with template variables

secrethub migrate config templates --var env=dev,staging,prod --var region=us-east-1,eu-west-1 my-config.yml.tpl

secrethub migrate config envfile --var env=dev,staging,prod --var region=us-east-1,eu-west-1 secrethub.env

Note: all config migration commands should be considered 'best effort'.