Add an Elastic Network Interface (ENI) to each Linux host, on a secondary subnet
we have provisioned in each AZ. Revise security groups as follows:
* Split out client security groups from servers so that we can't have clients
accidentally accessing serf addresses or other unexpected cross-talk.
* Add new security groups for the secondary subnet that only allows
communication within the security group so we can exercise behaviors with
multiple IPs.
This changeset doesn't include any Nomad configuration changes needed to take
advantage of the extra network interface. I'll include those with testing for
PR #16217.
Add a new driver capability: RemoteTasks.
When a task is run by a driver with RemoteTasks set, its TaskHandle will
be propagated to the server in its allocation's TaskState. If the task
is replaced due to a down node or draining, its TaskHandle will be
propagated to its replacement allocation.
This allows tasks to be scheduled in remote systems whose lifecycles are
disconnected from the Nomad node's lifecycle.
See https://github.com/hashicorp/nomad-driver-ecs for an example ECS
remote task driver.
