* Add rancher_host resource type
This adds a rancher_host resource type.
For now, the goal is to detect if the host already exists,
so that it can be purged cleanly when the host is deprovisioned.
The typical use is to create both an instance (e.g. aws_instance)
and a rancher_host resources with the same hostname. The rancher_host
resource will detect when the agent has registered itself.
When removing the host, both the instance and the rancher_host
resources can be removed, ensuring the host is purged from Rancher.
In future versions, this could support creating hosts as well.
* Use ro_labels to avoid removing internal Rancher labels
As reported in https://github.com/rancher/rancher/issues/8165
* Do not ForceNew on environment_id
When switching from one Rancher server to another, we want Terraform
to recreate Rancher resources. This currently leads to ugly `EOF` errors.
This patch resets resource Ids when they can't be found in the Rancher API.
