Add etcd key and cert environment variables for use with client auth

roles/kubernetes/master/tasks/pre-upgrade.yml

@@ -3,6 +3,8 @@
   command: "{{ bin_dir }}/etcdctl --peers={{ etcd_access_addresses }} ls /registry/minions"
   environment:
     ETCDCTL_API: 2
+    ETCDCTL_CERT_FILE: "{{ etcd_cert_dir }}/node-{{ inventory_hostname }}.pem"
+    ETCDCTL_KEY_FILE: "{{ etcd_cert_dir }}/node-{{ inventory_hostname }}-key.pem"
   register: old_data_exists
   delegate_to: "{{groups['etcd'][0]}}"
   changed_when: false