Support Default Provision Parameters at the class level

[carolynvs]

This is a follow-on to #2264 (see #2282 for how to implement it and what should be tested). It should add DefaultProvisionParameters to classes, and combine the class level params with the plan level params and the instance params.

This allows an operator to define defaults at the class level, tweak it at the plan level, and then a developer can use them (or override them) when they provision an instance of that class+plan.

[carolynvs]

#dibs