[Pacemaker] symmetric anti-collocation

[Alan Jones]

I've looked into the code more and added more logging, etc.
The pengine essentially walks the list of constraints, applying
weights, and then walks the list of resources and tallies the weights.
In my example, it ends up walking the resources backward, i.e. it
assigns a node to Y and then assigns a node to X.
Unfortunately, at the time of assigning a node to Y, X has no assigned
node and so the colocation rule cannot be applied.
What is needed is a backtracking method from the computer science area
of "constraint satisfaction".
Alan