Hardcoded AWS region

When making docker submissions the AWS region is hardcoded to `us-east-1`:

https://github.com/Cloud-CV/evalai-cli/blob/20fe1e61aa263aa8d16f93f67b24fefa6367c868/evalai/submissions.py#L124-L130

It remains possible to set arbitrary `AWS_REGION` values in eval-ai though:

https://github.com/Cloud-CV/EvalAI/blob/master/apps/challenges/utils.py#L66-L89

The issue specific to https://github.com/Cloud-CV/evalai-cli is:

1. How do you make a submission to a non `us-east-1` region?

A question that might need to be asked at https://github.com/Cloud-CV/EvalAI is:

2. If `challenge.use_host_credentials` is `True`, why are the challenge credentials used? My expectation was the host credentials referred to those stored in `os.environ`.

Start with evalai/submissions.py lines 124-130 in evalai-cli to trace the Docker submission region, then compare credential handling with apps/challenges/utils.py lines 66-89 in EvalAI. Done means the issue’s non-us-east-1 submission behavior and the use_host_credentials question are resolved and documented by the relevant tests or behavior.

Written by the indexing model from the issue text.