Limit the scope of cluster-update for heat drivers

[Spyros Trigazis]

At the moment cluster-update is only used to change the number
of nodes by applying a patch to the cluster object. Writing more
update methods will require to take the passed arguments and based
on the argument combination take the appropriate actions.

Another issue is that (for heat drivers) we pass all cluster template
parameters and the heat templates from the magnum controller host.
This is not desirable because long living clusters may break. A
cluster created in the N-2 release may be updated in the current
release where the heat templates and cluster-template parameters
are different.

What we can do is limit the scope of the update command (always for
heat drivers) use the existing parameters and pass only the node-count
to scale the cluster (in scale down pass the nodes to remove too).