FIx flannel var name

should be FLANNEL_ETCD_PREFIX

Related to: #2635

roles/flannel/tasks/main.yml

@@ -15,7 +15,7 @@
     - { regexp: "^(FLANNEL_ETCD=)", line: '\1{{ etcd_hosts|join(",") }}' }
     - { regexp: "^(FLANNEL_ETCD_ENDPOINTS=)", line: '\1{{ etcd_hosts|join(",") }}' }
     - { regexp: "^(FLANNEL_ETCD_KEY=)", line: '\1{{ flannel_etcd_key }}' }
-    - { regexp: "^(FLANNEL_ETCD_KEY_PREFIX=)", line: '\1{{ flannel_etcd_key }}' }
+    - { regexp: "^(FLANNEL_ETCD_PREFIX=)", line: '\1{{ flannel_etcd_key }}' }
 
 - name: Set flannel options
   become: yes