make private key optional

Setting private_key to null uses the local ssh-agent as a fallback for
authentication. Using the public_key instead of the private_key for
ssh -i lets the agent select the right identity if loaded. tested
with a yubikey

variables.tf

@@ -12,6 +12,7 @@ variable "public_key" {
 variable "private_key" {
   description = "SSH private Key."
   type        = string
+  default     = null
 }
 
 variable "location" {