use metal_ variables instead of equinix_metal_

[displague]

Reverts the equinix_metal_ variable names back to metal_.

metal_ is the current convention: https://github.com/equinix-labs/terraform-equinix-template/blob/main/variables.tf

---

Previously this PR was focused on the conversion to the Equinix provider, which was merged in #88

The old description follows:

Updates the Terraform provider from the Metal provider, which has been deprecated, to the Equinix provider.

If updating from a previously deployed environment, the following guide should help: https://registry.terraform.io/providers/equinix/equinix/latest/docs/guides/migration_guide_equinix_metal

terraform apply builds the HA Anthos 1.8.7 environment successfully.

$ terraform init
...
$ terraform validate
...
$ terraform apply
...
Apply complete! Resources: 58 added, 0 changed, 0 destroyed.

Outputs:

Control_Plane_Public_IPs = [
  "147.28.148.43",
  "147.28.148.137",
  "147.28.148.111",
]
Control_Plane_VIP = "145.40.81.128"
Equinix_Metal_Project_ID = "24822e74-bf8c-4637-bcb9-71ef8fd88eed"
Ingress_VIP = "147.28.154.43"
Kubeconfig_location = "/Users/marques/src/terraform-metal-anthos-on-baremetal/eqnx-metal-gke-tgll8-kubeconfig"
Worker_Public_IPs = [
  "147.28.148.61",
  "147.28.148.63",
  "147.28.148.37",
]
ssh_key_location = "/Users/marques/.ssh/anthos-eqnx-metal-gke-tgll8"

$ KUBECONFIG=/Users/marques/src/terraform-metal-anthos-on-baremetal/eqnx-metal-gke-tgll8-kubeconfig kubectl get nodes 
NAME                             STATUS   ROLES                  AGE   VERSION
eqnx-metal-gke-tgll8-cp-01       Ready    control-plane,master   16m   v1.20.9-gke.102
eqnx-metal-gke-tgll8-cp-02       Ready    control-plane,master   10m   v1.20.9-gke.102
eqnx-metal-gke-tgll8-cp-03       Ready    control-plane,master   10m   v1.20.9-gke.102
eqnx-metal-gke-tgll8-worker-01   Ready    <none>                 14m   v1.20.9-gke.102
eqnx-metal-gke-tgll8-worker-02   Ready    <none>                 14m   v1.20.9-gke.102
eqnx-metal-gke-tgll8-worker-03   Ready    <none>                 14m   v1.20.9-gke.102

[displague]

I enabled HA and #83 appears to not be an issue.