Fix default constraint values when scaling units.

Also fix the memory unit used (s/GB/MB).

QA:
- make devel;

- deploy a service, do not change constraints;
- scale up the service: you should see the constraints
  message indicating the default constraints will be used;

- refresh and deploy another service customizing constraints;
- scale up the service: you should see the message indicating
  constraints are set for this service, including CPU power and
  number of cores.