Description of problem:
When override is ticked for a smart class parameter, the value is always supplied to Puppet - be it a default value, or an override.  Setting a default should be optional, in which case no value is given to Puppet via the ENC unless an override is used.

This will allow the default value of the parameter to be determined by the Puppet manifest rather than having to be set to a fixed value in Foreman.

Version-Release number of selected component (if applicable):
MDP2

How reproducible:
Always

Steps to Reproduce:
For an example_class with a parameter called example_param, whose default value is "foo":

1. Configure > Puppet class, edit a class' smart class parameter and tick Override
2. Edit or create a host, assign the class to it
3. View the host page, click YAML

Actual results:
classes:
  example_class:
    example_param: foo

Expected results:
classes:
  example_class