custom variables

• Injecting custom variables from the cli with --custom-variable KEY=VALUE (can be specify multiple times) or from the env description with the "custom_variables" hash or in the cluster configuration with the hash of the same name.
• The custom variables are accessible in kadeploy scripts by prefixing them by "KADEPLOY_". (eg: KADEPLOY_DEBUG)
• The name of the variable and the value must be a string.
• Priority of variables: CLI > env description > cluster configuration. A warning appears when a custom variable is overwrite. (eg: "Custom Variables from the environement overwrite by the client: DEBUG.")
• Some name are reserved by kadeploy and can't be overwrite. An error is thrown when trying to name a custom variable with such name from the CLI. (eg: "You cannot overwrite Kadeploy variables: CLUSTER.")

TODO:

• Ajouter l'option dans kareboot (et kapower?)
• Meilleur gestion de l'erreur où la variable existe déjà
• Mieux traiter les inputs (pour l'instant je vérifie juste que c'est un hash, mais il faut veriier que c'est un hash que d'une profondeur et avec que des strings ?)