Possible actions on an instance.
Actions should probably match a user intention at the API level. Because they can be user visible that should help to avoid confusion. For that reason they tend to maintain the casing sent to the API.
Maintaining a list of actions here should protect against inconsistencies when they are used.
The naming style of instance actions should be snake_case, as it will consistent with the API names. Do not modify the old ones because they have been exposed to users.