add tests and update readme for apigee psc peering

tests/modules/apigee/organization_only_vpc_mode.tfvars

@@ -0,0 +1,10 @@
+project_id = "my-project"
+organization = {
+  display_name            = "My Organization"
+  description             = "My Organization"
+  authorized_network      = "my-vpc"
+  runtime_type            = "CLOUD"
+  billing_type            = "PAYG"
+  database_encryption_key = "123456789"
+  analytics_region        = "europe-west1"
+}