Make the extension(s) that we don't want to remove configurable

[Michael-Sinz]

The extension(s) that we may wish to not remove from the VMSS after prototype model may change or be different for certain cases.

We have a scenario with a security extension that still wishes to run on new nodes even if the node already has the extension installed and running.

Currently we have one that we keep "just because" but that should also be configurable as it may be different in other scenarios.