packer: add default to aws auth variables

So you don't have to pass these if packer is supposed to find them
on its own (instance profile, local profile).

Signed-off-by: Ondřej Budai <ondrej@budai.cz>

templates/packer/variables.pkr.hcl

@@ -2,8 +2,14 @@
 # when running `packer build`
 
 # AWS account configuration.
-variable "aws_access_key" { type = string }
-variable "aws_secret_key" { type = string }
+variable "aws_access_key" {
+  type = string
+  default = ""
+}
+variable "aws_secret_key" {
+  type = string
+  default = ""
+}
 variable "region" {
   type    = string
   default = "us-east-1"