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/etcd/meta/main.yml

@@ -0,0 +1,7 @@
+---
+dependencies:
+  - role: adduser
+    user: "{{ addusers.etcd }}"
+    when: ansible_os_family != 'CoreOS'
+  - role: download
+    file: "{{ downloads.etcd }}"