Don't write docker proxy config if docker_mirror undef

If docker_mirror isn't set, we set one. But if we do it when mirror_fqdn
isn't set, we get a bogus value. Put in guards for that so that we don't
write out bad values.

Change-Id: Ie56d9a83fbabcc99420ce28a8a8f1bb562db0470

roles/install-docker/tasks/mirror.yaml

@@ -7,6 +7,7 @@
 - name: Set docker_mirror fact for upstream docker
   when:
     - docker_mirror is not defined
+    - mirror_fqdn is defined
     - use_upstream_docker
   set_fact:
     docker_mirror: "http://{{ mirror_fqdn }}:8082"
@@ -14,12 +15,15 @@
 - name: Set docker_mirror fact for distro docker
   when:
     - docker_mirror is not defined
+    - mirror_fqdn is defined
     - not use_upstream_docker
   set_fact:
     docker_mirror: "http://{{ mirror_fqdn }}:8081/registry-1.docker/"
 
 - name: Install dockerhub proxy configuration
   become: yes
+  when:
+    - docker_mirror is defined
   template:
     dest: /etc/docker/daemon.json
     group: root