Add environment variables to force abd order physical devices

[christophe-lunarg]

Vulkan Configurator allows to force a physical device and sort physical devices however using Vulkan Configurator is not convenient for people running script based tests, using environment variables and case we discussed with @artem-lunarg.

It would be great to add environment variables to force a physical device easily. There is currectly a way using the json driver filename which is not trivial to figure out even for Vulkan Configurator

• A first improvment could be to have a force physical driver environment variable that can take an integer identifying the physical device in the list. Sure, the list can change order but this is a rare occurance.
• A second improvement could be to use an identifier that could trivially be generated with Vulkan Configurator and copy pasted to the environment variable.
• A third improvment could be another environment variable used to sort the physical device. If a physical device is not in the list, it's not visible to the Vulkan applications. (matching Vulkan Loader settings file behavior)