Make Rancher server URL configurable

Change-Id: I19fbd3e5cb71e4056b16ffac5da611e3039995a9
diff --git a/aether-playbook/roles/k8s/defaults/main.yml b/aether-playbook/roles/k8s/defaults/main.yml
index 6132d2f..e64dccd 100644
--- a/aether-playbook/roles/k8s/defaults/main.yml
+++ b/aether-playbook/roles/k8s/defaults/main.yml
@@ -30,3 +30,5 @@
     - registry.central.aetherproject.net
 
 docker_version: 18.06.3~ce~3-0~ubuntu
+
+rancher_server_url: https://aether.onlab.us