Reconcile new variables against user inventory

[scottd018]

Problem:

As we progress RPK, we have no way of working with old inventory files. When new variables get added, users are unaware of it so their automation breaks. Backwards compatibility does not exist in these scenarios.

Idea:

Have a support role that has a template in it
Use build/inventory as the variables
Create the new inventory from the template and backup the old file for the user

make update.inventory perhaps could be the target