providers/heroku: config vars aren't computed, they're non-exclusive

/cc @pearkes - See doc change

builtin/providers/heroku/resource_heroku_app.go

@@ -69,7 +69,6 @@ func resourceHerokuApp() *schema.Resource {
 			"config_vars": &schema.Schema{
 				Type:     schema.TypeList,
 				Optional: true,
-				Computed: true,
 				Elem: &schema.Schema{
 					Type: schema.TypeMap,
 				},
@@ -141,6 +140,19 @@ func resourceHerokuAppRead(d *schema.ResourceData, meta interface{}) error {
 		return err
 	}
 
+	// Only get the config vars that we care about
+	care := make(map[string]struct{})
+	for _, v := range d.Get("config_vars").([]interface{}) {
+		for k, _ := range v.(map[string]interface{}) {
+			care[k] = struct{}{}
+		}
+	}
+	for k, _ := range app.Vars {
+		if _, ok := care[k]; !ok {
+			delete(app.Vars, k)
+		}
+	}
+
 	d.Set("name", app.App.Name)
 	d.Set("stack", app.App.Stack.Name)
 	d.Set("region", app.App.Region.Name)

website/source/docs/providers/heroku/r/app.html.markdown

@@ -32,8 +32,10 @@ The following arguments are supported:
 * `stack` - (Optional) The application stack is what platform to run the application
    in.
 * `config_vars` - (Optional) Configuration variables for the application.
-   This is a map that can set keys against the application.
+     The config variables in this map are not the final set of configuration
-
+     variables, but rather variables you want present. That is, other
+     configuration variables set externally won't be removed by Terraform
+     if they aren't present in this list.
 
 ## Attributes Reference