Get an AWS session in the same way as Terraform

[wata727]

Fixes #134

This pull request introduces github.com/hashicorp/aws-sdk-go-base and gets an AWS session in the same way as Terraform.

Also, it will take provider blocks in configuration files into account. For example, the following region settings are also propagated to TFLint, and redundant region settings are not required:

provider "aws" {
  region = "us-east-1"
}

However, this results in changing the priority of the credentials. This is an incompatible change. For example, regions and static credentials in the configuration file have higher priority than environment variables, so you need to set static credentials as a TFLint configuration. See the changed documentation for the new priorities.

In addition, ECS (CodeBuild) task roles and EC2 roles were supported. Please note that Assume role is not supported.