Add region property to DAG

[spencerseale]

Add DAG.region for better detection of DAG host region

[spencerseale]

The only issue I foresee is if someone overrides region after DAG already started. DAG.region as it's implemented here, gets current region. The alternative would be to return a non-NoneType only after DAG.compute is fired. Since a DAG is only usable for one task graph execution I see this alternative as more validation after running. However the original inspiration for this was to validate region prior to executing DAG.

[sgillies]

@spencerseale if I read the code correctly, the new property doesn't tell us for sure what region the DAG is running in, it tells us the current client configuration. Which is pretty much what you said above, right?

I think this goes against the OO principle of encapsulation. DAG methods should be based on DAG data, not data in another class or module.

[spencerseale]

@sgillies That's correct and you're right. I can take another stab at it so it's pinned to the DAG object.