Make Rancher server URL configurable

Change-Id: I19fbd3e5cb71e4056b16ffac5da611e3039995a9
diff --git a/aether-playbook/roles/k8s/tasks/k8s.yml b/aether-playbook/roles/k8s/tasks/k8s.yml
index 1fc73f6..1d3e90c 100644
--- a/aether-playbook/roles/k8s/tasks/k8s.yml
+++ b/aether-playbook/roles/k8s/tasks/k8s.yml
@@ -42,7 +42,7 @@
     --volume /etc/kubernetes:/etc/kubernetes
     --volume /var/run:/var/run
     rancher/rancher-agent:v2.3.3
-    --server https://rancher.central.aetherproject.net
+    --server {{ rancher_server_url }}
     --token {{ rancher_cluster_token }}
     --ca-checksum {{ rancher_ca_checksum }}
     {{ k8s_roles }}