Add example for using external resources

Add template for show cases that how we can use external resources.
external resources in stack does not required to be modified or depends
on the stack.
blueprint external-resources

Depends-On: I8b33c957a7185a0f52356f412b13609ba1a7b469
Change-Id: Ia56318508caf43a7da28bd479becdc602477cad5

hot/external_resource.yaml

@@ -0,0 +1,21 @@
+heat_template_version: newton
+
+description: >
+  This template shows use case for external resource.
+  external resource can be any resource (create by
+  heat or not) that heat got type for it.
+
+parameters:
+  external_server_id:
+    type: string
+    default: da5b940a-14ff-4884-8e95-5cf6a3a1028f
+    description: ID of external server
+
+resources:
+  external_server:
+    type: OS::Nova::Server
+    external_id: { get_param: external_server_id }
+
+outputs:
+  server_address:
+    value: { get_attr: [external_server, networks, private, 0 ] }