Error out on too few nodes

If a deployment with too few nodes is attempted,
show a helpful error message, and don't try to create a stack.

The same also goes for scaling - if scaling is attempted and
there are not enough nodes, error out and don't create a stack.

Change-Id: I367ced64f31b818dfa474745dc96c02dacebc65d
Merge "Error out on too few nodes"