Allow to provide AWS credentials in backend configuration

As an alternative to using an IAM role or environment variables.

Inspired by https://github.com/fanduel/hiera-cloudformation/blob/75c92193e06a2baf45b75801fc8e425ba6a2d144/lib/hiera/backend/cloudformation_backend.rb#L34-L43
Wordsmithing
Simplify check for Config attributes
Merge pull request #2 from Jimdo/aws-credentials

Allow to provide AWS credentials in backend configuration