[stress testing][m] Support multiple regions

Long term testing will sometimes require running in separate regions (localizing both the code and the created resources).

You can kind of hack this together today by hardcoding a location in your bicep file and asking @benbp /@ckairen to create a nodepool in a particular region) but there should be a way to indicate this at the stress test level.

This is a requirement two ways - some regions appear to have different reliability/performance and sometimes some regions will have software/features that don't exist in other regions as well.

This could also help us if we want to spread our resources out a bit and alleviate pressure on whichever region we're deploying to today.

CC: @ckairen , @benbp