Remove internal providers

[ofhouse]

Follow up from #49.

We currently define an internal provider alias for the aws provider to force the creation of some resources in us-east-1 region.
This behaviour is generally not recommended by Terraform and can cause trouble for users that work with different AWS account profiles.
e.g. you have a private (default) and work profile (alias), the internal provider would always use the default profile even when you explicitly initialize the aws provider with the alias profile.

Our initial approach here was to reduce configuration effort since the us-east-1 alias must be created and passed by the user into the module but we should follow the official recommendation from Terraform here and move it into userland.

[ofhouse]

Terraform v0.15 introduces a new option configuration_aliases in the required_provider block for this: https://github.com/hashicorp/terraform/blob/v0.15/website/upgrade-guides/0-15.html.markdown#alternative-provider-configurations-within-modules