default=null for shared_vpc_self_link and vpc_host_project (#502)

* Update variables.tf

* Update README.md

fast/stages/03-project-factory/prod/README.md

@@ -108,11 +108,11 @@ terraform apply
 | name | description | type | required | default | producer |
 |---|---|:---:|:---:|:---:|:---:|
 | [billing_account_id](variables.tf#L19) | Billing account id. | <code>string</code> | ✓ |  | <code>00-bootstrap</code> |
-| [shared_vpc_self_link](variables.tf#L44) | Self link for the shared VPC. | <code>string</code> | ✓ |  | <code>02-networking</code> |
-| [vpc_host_project](variables.tf#L50) | Host project for the shared VPC. | <code>string</code> | ✓ |  | <code>02-networking</code> |
 | [data_dir](variables.tf#L25) | Relative path for the folder storing configuration data. | <code>string</code> |  | <code>&#34;data&#47;projects&#34;</code> |  |
 | [defaults_file](variables.tf#L38) | Relative path for the file storing the project factory configuration. | <code>string</code> |  | <code>&#34;data&#47;defaults.yaml&#34;</code> |  |
 | [environment_dns_zone](variables.tf#L31) | DNS zone suffix for environment. | <code>string</code> |  | <code>null</code> | <code>02-networking</code> |
+| [shared_vpc_self_link](variables.tf#L44) | Self link for the shared VPC. | <code>string</code> |  | <code>null</code> | <code>02-networking</code> |
+| [vpc_host_project](variables.tf#L51) | Host project for the shared VPC. | <code>string</code> |  | <code>null</code> | <code>02-networking</code> |
 
 ## Outputs
 

fast/stages/03-project-factory/prod/variables.tf

@@ -45,10 +45,12 @@ variable "shared_vpc_self_link" {
   # tfdoc:variable:source 02-networking
   description = "Self link for the shared VPC."
   type        = string
+  default     = null
 }
 
 variable "vpc_host_project" {
   # tfdoc:variable:source 02-networking
   description = "Host project for the shared VPC."
   type        = string
+  default     = null
 }