add "reset" option to DeviceClassSpec (ZPS-810)

[j053ph4]

• Added optional "reset" parameter to facilitate override of zProperties on preexisting device class
• Added test_device_class_existing_zproperties unit test

[joshw]

I think this needs a doc update as well. This seems like a feature that we would only rarely want to use, so it's important to point out why it's potentially a bad idea to use it, and where it is appropriate.

[j053ph4]

I updated the docs per your suggestion along with a cautionary tale about bad stuff that could happen. I only listed one scenario while indicating others exist and that migrate scripts are the way to go

[j053ph4]

Thanks Josh!