Add Terraform v0.11.x support and migration docs

* Add explicit "providers" section to modules for Terraform v0.11.x
* Retain support for Terraform v0.10.4+
* Add migration guide from Terraform v0.10.x to v0.11.x for those managing
existing clusters (action required!)

docs/topics/maintenance.md

@@ -127,3 +127,78 @@ Typhoon supports multi-controller clusters, so it is possible to upgrade a clust
 !!! warning
     Typhoon does not support or document node replacement as an upgrade strategy. It limits Typhoon's ability to make infrastructure and architectural changes between tagged releases. 
 
+## Terraform v0.11.x
+
+Terraform v0.10.x to v0.11.x introduced breaking changes in the provider and module inheritance relationship that you MUST be aware of when upgrading to the v0.11.x `terraform` binary. Terraform now allows multiple named (i.e. aliased) copies of a provider to exist (e.g `aws.default`, `aws.somename`). Terraform now also requires providers be explicitly passed to modules in order to satisfy module version contraints (which Typhoon modules define). Full details can be found in [typhoon#77](https://github.com/poseidon/typhoon/issues/77) and [hashicorp#16824](https://github.com/hashicorp/terraform/issues/16824).
+
+In particular, after upgrading to the v0.11.x `terraform` binary, you'll notice:
+
+* `terraform plan` does not succeed and prompts for variables when it didn't before
+* `terraform plan` does not succeed and mentions "provider configuration block is required for all operations"
+* `terraform apply` fails when you comment or remove a module usage in order to delete a cluster
+
+### New users
+
+New users can start with Terraform v0.11.x and follow the Typhoon docs without issue.
+
+### Existing
+
+Users who used modules to create clusters with Terraform v0.10.x and still manage those clusters via Terraform must explicitly add each provider used in `provider.tf`:
+
+```
+provider "local" {
+  version = "~> 1.0"
+  alias = "default"
+}
+
+provider "null" {
+  version = "~> 1.0"
+  alias = "default"
+}
+
+provider "template" {
+  version = "~> 1.0"
+  alias = "default"
+}
+
+provider "tls" {
+  version = "~> 1.0"
+  alias = "default"
+}
+```
+
+Modify the `google`, `aws`, or `digitalocean` provider section to specify an explicit `alias` name.
+
+```
+provider "digitalocean" {
+  version = "0.1.2"
+  token = "${chomp(file("~/.config/digital-ocean/token"))}"
+  alias = "default"
+}
+```
+
+!!! note
+    In these examples, we've chosen to name each provider "default", though the point of the Terraform changes is that other possibilities are possible.
+
+Edit each instance (i.e. usage) of a module and explicitly pass the providers.
+
+```
+module "aws-cluster" {
+  source = "git::https://github.com/poseidon/typhoon//aws/container-linux/kubernetes"
+  
+  providers = {
+    aws = "aws.default"
+    local = "local.default"
+    null = "null.default"
+    template = "template.default"
+    tls = "tls.default"
+  }
+
+  cluster_name = "somename"
+```
+
+Re-run `terraform plan`. Plan will claim there are no changes to apply. Run `terraform apply` anyway as this will update Terraform state to be aware of the explicit provider versions.
+
+### Verify
+
+You should now be able to run `terraform plan` without errors. When you choose, you may comment or delete a module from Terraform configs and `terraform apply` should destroy the cluster correctly.