Hardcoded AWS region

[masonlr]

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.