0.15.0 update aws provider version

what

• Update aws provider to >= 2.0 avoid module version conflicts when used with >= 3.0 modules (eg. aws-eks-node-group).

why

• This module is being used with terraform-aws-eks-node-group which needs aws provider version to be >= 3.0 which is causing version conflicts.

references

• See cloudposse/terraform-aws-eks-node-group#41 for details. This tackles one of the modules causing the issue.