Define empty default objects for docker package management.

On CoreOS where there is no package management, perform zero-trip
loops instead of throwing an exception for iterating over a member
of an undefined variable.

roles/docker/defaults/main.yml

@@ -1 +1,10 @@
 docker_version: 1.10
+
+docker_package_info:
+  pkgs:
+
+docker_repo_key_info:
+  repo_keys:
+
+docker_repo_info:
+  repos: