Move k8s install to pre playbook

In the k8s integration job, run the install-kubernetes role in
the pre-playbook since it is not (generally) the thing under test.

Change-Id: Ib20cd12100f323f1eb262261394c4b180dc1f2cd

playbooks/nodepool-functional-k8s/pre.yaml

@@ -1,6 +1,8 @@
 - hosts: all
   roles:
     - role: bindep
+    - role: install-kubernetes
+      docker_version: 18.06.1~ce~3-0~ubuntu
 
   tasks:
     - name: Ensure nodepool services directories

playbooks/nodepool-functional-k8s/run.yaml

@@ -1,6 +1,4 @@
 - hosts: all
   roles:
-    - role: install-kubernetes
-      docker_version: 18.06.1~ce~3-0~ubuntu
     - role: tox
       tox_envlist: functional_kubernetes