Make Rancher optional in the installation process

Restructure as modules following best practices: https://www.terraform.io/docs/registry/modules/publish.html#requirements

Create module usage examples:

• rancher_eks: working as is

• provided_eks: using provided eks

• eks: creating eks in terraform via https://github.com/terraform-providers/terraform-provider-aws/tree/master/examples/eks-getting-started or an existing module like https://github.com/howdio/terraform-aws-eks

Extra requirements from the field (in priority order):

• optional content service

• optional tiller setup

• external docker registry support

• optional EFS and other AWS resources

• optional AWS certificate generation and DNS

• removal of wildcard DNS as not supported by azure