data: Docs for encrypted volumes for compute machines

Partially reverts 0c370dd (data/aws: Encrypt the AMI used by the
bootstrap and master machines, 2019-02-22, #1296).  This isn't a clean
revert; for example, I left the ability to destroy images which are
tagged as owned by the cluster.  And we're still copy-and-encrypting
for the bootstrap machine and control-plane machines until the AWS
Terraform provider supports requesting encrypted root volumes [1].
But with this commit, we're now documenting the encryption in a way
that covers both the previous AMI-based encryption used for
bootstrap/control-plane and the new root-volume-based encryption used
for the compute machines, because they come down to encrypted root
volumes regardless of their approach.

[1]: hashicorp/terraform-provider-aws#8624

data/data/aws/variables-aws.tf

@@ -19,7 +19,7 @@ variable "aws_master_instance_type" {
 
 variable "aws_ami" {
   type = string
-  description = "AMI for all nodes.  An encrypted copy of this AMI will be used.  Example: `ami-foobar123`."
+  description = "AMI for all nodes.  Example: `ami-foobar123`."
 }
 
 variable "aws_extra_tags" {

docs/user/aws/install.md

@@ -49,12 +49,9 @@ In EC2, there will be 6 running instances:
 
 ![EC2 instances after install](images/install_nodes.png)
 
-The installation creates an encrypted AMI for the bootstrap and control-plane machines.
-The encrypted AMI is [copied][encrypted-copy] from the AMI configured in the control-plane machine-API provider spec,
-which is RHCOS by default.
+Instance root volumes for the bootstrap and contro-plane machines are encrypted.
 The encryption uses the default EBS key for your target account and region
 (`aws kms describe-key --key-id alias/aws/ebs`).
-The encrypted AMI is deregistered by `destroy cluster`.
 
 An architecture diagram for the AWS elements created by the full installation is as depicted:
 
@@ -69,4 +66,3 @@ The OpenShift console is available via the kubeadmin login provided by the insta
 ![OpenShift web console](images/install_console.png)
 
 [cloud-install]: https://cloud.openshift.com/clusters/install
-[encrypted-copy]: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/AMIEncryption.html#create-ami-encrypted-root-snapshot