Goal: 

• 
  The fencer should support a "cancel fence action request" API call that could be honored if (1) a fencing attempt in progress fails and would otherwise be retried; or (2) a fencing attempt is in the delay period of a device configured with a delay. Note that if any part of a topology-based fence action has succeeded, no part of the action should be cancelled.

stonith_admin should support a new option to send the cancellation (mainly so we can test the feature)

Acceptance Criteria

• stonith_admin cancel option can be used while a fence action is in progress, and if the fence action fails, it will not be retried
• stonith_admin cancel option can be used while a fence action has been requested but has a delay that has not yet expired, and the action will not be executed