metal: add tectonic_ssh_private_key variable

Allow users to specify the SSH private key via a variable in cases where
ssh-agent isn't available or configured.

platforms/metal/remote.tf

@@ -6,6 +6,7 @@ resource "null_resource" "kubeconfig" {
     host    = "${element(concat(var.tectonic_metal_controller_domains, var.tectonic_metal_worker_domains), count.index)}"
     user    = "core"
     timeout = "60m"
+    private_key = "${var.tectonic_ssh_private_key}"
   }
 
   provisioner "file" {
@@ -32,6 +33,7 @@ resource "null_resource" "bootstrap" {
     host    = "${element(var.tectonic_metal_controller_domains, 0)}"
     user    = "core"
     timeout = "60m"
+    private_key = "${var.tectonic_ssh_private_key}"
   }
 
   provisioner "file" {

platforms/metal/variables.tf

@@ -185,3 +185,9 @@ SSH public key to use as an authorized key.
 Example: `ssh-rsa AAAB3N...`
 EOF
 }
+
+variable "tectonic_ssh_private_key" {
+  type        = "string"
+  description = "SSH private key corresponding to tectonic_ssh_authorized_key. If not provided, SSH agent will be used."
+  default     = ""
+}