instance: fix round-trip serialization

[josephholsten]

builds on #151, attempting to fix #113

[codycushing]

Setting up an instance with a plan then running apply again without any change causes the instance to be destroyed and recreated--when it diffs it thinks the subnet is different for some reason even though it's exactly the same.

[josephholsten]

@codycushing what config are you using? I'm not seeing that with the config in the acc test:

resource "baremetal_core_instance" "t" {
  availability_domain = "${data.baremetal_identity_availability_domains.ADs.availability_domains.0.name}"
  compartment_id      = "${var.compartment_id}"
  image               = "` + FastestImageID + `"
  shape               = "` + SmallestShapeName + `"
  subnet_id           = "${baremetal_core_subnet.s.id}"
}

[codycushing]

It's with a plan using the nested vnic object:

resource "baremetal_core_instance" "instance1" {
  availability_domain = "${lookup(data.baremetal_identity_availability_domains.ADs.availability_domains[0],"name")}" 
  compartment_id = "${var.compartment_ocid}"
  display_name = "be-instance1"
  image = "${lookup(data.baremetal_core_images.image-list.images[0], "id")}"
  shape = "VM.Standard1.1"

  create_vnic_details {
    subnet_id = "${var.subnet}"
    display_name = "vnic1"
    assign_public_ip = true
    hostname_label = "be-instance1"
  },
}

[josephholsten]

@codycushing I see what's happening now!

essentially, after the instance is created, the impl is wanting to set resource.baremetal_core_instance.instance1.subnet_id from the response. I was hoping https://github.com/oracle/terraform-provider-baremetal/pull/159/files#diff-930932b2c63a550ddecdf09c110553bdR242 would keep that from breaking things, but I'd actually need to hack the state (and violate layering) in order to trick terraform into thinking that a null resource.baremetal_core_instance.instance1.subnet_id doesn't need to change.

Talking with @grubernaut, it looks like it's not really feasible to support the old resource.baremetal_core_instance.instance1.subnet_id attribute and the new resource.baremetal_core_instance.instance1.create_vnic_details.0.subnet_id in the same impl, so we need a backwards incompatible change.

While we're at it, I'd really like to rename the create_vnic_details attribute to vnic, which makes more sense when reading attributes, and will be more clear when (if?) instances support more than one vnic.

[josephholsten]

I'm going to create a variant of this PR that is backward incompatible, but has state migration so no data (or resources) are broken