Improve RHEV fencing robustness by retrying failed fence attempts

Issue

Currently when a RHEV host becomes unresponsive and has to be fenced only one other host (the "fence proxy") within the cluster is responsible of fencing it.

The request here is to improve the robustness of fencing. If a fencing attempt fails, e.g. re-attempt the fencing action, and/or attempt it from a different host. E.g. the "fence proxy" might have some connectivity problems to the victim's power management system, but it could well be that other hosts can access it and succeed at fencing.