turn adduser/download roles into meta roles

This should make things a little more composable,
by making these roles meta roles that perform no
actions by default we allow each role to own its own
resources.

roles/network_plugin/weave/meta/main.yml

@@ -0,0 +1,4 @@
+---
+dependencies:
+  - role: download
+    file: "{{ downloads.weave }}"