Add local/offline support

[alexcasalboni]

The module could provide a way to avoid API calls to AWS/SSM when working locally.

For example, we could use os.environ to look for values instead of querying the Parameter Store. Such a functionality would speed up local development but at the same time it'd make the local environment different from production (e.g. latency, potential network errors, etc.).

[benkehoe]

I'd suggest also making the boto3 client getting configurable, to use a library like placebo An example: https://github.com/iRobotCorporation/cfn-custom-resource/blob/master/cfn_custom_resource/cfn_custom_resource.py#L231