API extension: /servers/{id}/actions

Currently, the /servers/{id}/action supports a number of actions such as SUSPEND, UNSUSPEND, POWER_ON, etc. These actions must be known to the client application to be used. To make it simpler, I'm proposing that we add a new …/actions resource that would enumerate the actions available.

In addition, if security settings are available, the …/actions resource should only enumerate those actions that are available to the currently authenticated user. (This may be a future feature.)