Add ECS Cluster Region option

[galindro]

I need to run an ECS instance in a different region from the ECS cluster. So, scope needs to talk with the correct cluster region to get cluster tasks. But, as we could see here, the method newClient takes the region from the instance metadata.

It would be nice to scope get this information from a command line argument. This way, we can make scope sufficiently flexible to be used in this use-case.

This PR covers the proposed behaviour. If the user don't inform the ECS Cluster region, it get's from instance metadata by default. I've successfully built and test it over my environment.

[galindro]

merged