Port over namespace modules from terraform-kubernetes-helm

[yorinasub17]

This ports over the namespace modules from terraform-kubernetes-helm to this new repo. The modules have also been renamed to drop redundancy:

• k8s-namespace => namespace
• k8s-namespace-roles => namespace-roles
• k8s-service-account => service-account

The following changes were also introduced as a part of this porting:

• The modules now work with terraform 0.13.x.
• Resource rename: rbac_tiller_metadata_access_role => rbac_helm_metadata_access_role
• Resource rename: rbac_tiller_resource_access_role => rbac_helm_resource_access_role

[brikis98]

I assume most of this code is moved as-is from the original repo, so I didn't do too deep of a review, but from a quick scan of the new stuff, this LGTM!

[yorinasub17]

Thanks for the review! Going to merge this now since the tests are passing and will draft the release notes + migration guide.