Add tests for AWS Cloudwatch

[onno-vos-dev]

Description

This issue aims to solve AWS Cloudwatch testing specifically as it is likely one of the most used modules of aws-erlang.

See the milestone for further details.

Acceptance Criteria

An aws_cloudwatch_SUITE is created which can talk to a real AWS Account and perform certain AWS Requests towards it. A subset of the functionality should be tested to start with.

• get_metric_data