Add fusion support to azure batch executor

[bentsherman]

Fusion support for the azure batch task handler is trickier because much of the task logic (container options, environment vars, submit command) are handled in a separate class AzBatchService. So we either need to pass the executor and script launcher all the way up the call stack or move some of the fusion methods into FusionHelper.

I'm currently trying the latter approach to see how it works. So AzBatchTaskHandler doesn't implement the FusionAwareTask trait at all, it just calls helper functions from FusionHelper.

Still need to update unit tests and test on azure.