[services] Tunnelbroker - Get AWS region from aws/config only.

Summary:
Switch to use the AWS region from ~/.aws/config file instead of looking it at the ~/.aws/credentials first.
The main reason for that, AWS CLI sets the region inside the ~/.aws/config file and we need both config and credentials to follow the AWS CLI approach.

Linear task: ENG-559

Test Plan:
Tunnelbroker successfuly connect to AWS using ~/.aws/credentials for credentials and ~/.aws/config for the region:

cat ~/.aws/config
[default]
region = us-east-2

cat ~/.aws/credentials
[default]
aws_access_key_id = .....
aws_secret_access_key = .....

Reviewers: karol-bisztyga, palys-swm, varun, jimpo, ashoat

Reviewed By: karol-bisztyga, jimpo, ashoat

Subscribers: benschac, ashoat, palys-swm, Adrian, atul, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3119