Image create and set command updates and tests

Refactor image create and set commands to properly handle properties.
This is particularly tricky with exclusive booleans as in this case leaving
both choices off the command line should NOT assume a default value but
leave the existing value unchanged.

Properties were not being updated correctly in the 'image set' command.
Refactor it to use the same pattern as in other SetXxx commands.
Add tests for arg handling.

Change-Id: I123a64c9b4feecab25a3e2013cc047f55b1c9967