Use default subnet and SG names

If no subnet_id is specified and a default subnet exists in the Default
VPC, the instance will be automatically created in the default subnet.

Using SG names instead of IDs should also resolve the issue where
`terraform plan` is not clean after a cranehost is created.

See this issue for additional details: hashicorp/terraform-provider-aws#1993

main.tf

@@ -19,8 +19,7 @@ resource "aws_instance" "instance" {
   ami = "${coalesce(var.ami, data.aws_ami.ami.image_id)}"
   instance_type = "${var.instance_type}"
   key_name = "${var.ssh_key_name}"
-  subnet_id = "${var.subnet_id}"
-  security_groups = ["${aws_security_group.sg.id}"]
+  security_groups = ["${aws_security_group.sg.name}"]
   associate_public_ip_address = true
 
   tags {

variables.tf

@@ -15,10 +15,6 @@ variable region {
   default = "eu-west-1"
 }
 
-variable subnet_id {
-  description = "Subnet Id (default for buildo: subnet-789e130f, omnilab: subnet-13126d4a)"
-}
-
 variable volume_size {
   description = "Volume size"
   default = 8
@@ -60,4 +56,4 @@ variable disk_utilization_alarm_threshold {
 
 variable bellosguardo_target {
   description = "Possible values are 'buildo', 'omnilab'"
-}
+}