The overbook ratio is now a part of the CShareableResource view.

This makes the Overbook constraint _finally_ optional. By default, a value of 1 is assumed.
Oops
Fix regressions.

In practice, as there is a default overbooking factor, we are back to the situation in entropy were the resource usage made by the VM cannot exceed the node capacity.
minor documentation
Missing apidocs
better constraints naming
update GettingStarted to remove the useless Overbook constraints