image-serve: only uninstall docker-distribution when it was installed

Change-Id: Ib34d2ac3d6cb53cc40ec116953338568d1fbc6cd

deployment/image-serve/image-serve-baremetal-ansible.yaml

@@ -63,13 +63,21 @@ outputs:
           - include_role:
               name: tripleo-image-serve
       upgrade_tasks:
-        - name: Stop, disable docker-distribution
+        - name: Uninstall docker-distribution
           when: step|int == 3
+          block:
+            - name: check if docker-distribution is installed
+              command: /usr/bin/rpm -q docker-distribution
+              failed_when: false
+              register: docker_distribution_installed
+              check_mode: no
+            - name: Stop, disable docker-distribution
               systemd:
                 enabled: false
                 state: stopped
                 name: docker-distribution
+              when: docker_distribution_installed.rc == 0
             - name: Uninstall docker-distribution
-          when: step|int == 3
               package: name=docker-distribution state=absent
+              when: docker_distribution_installed.rc == 0
             # TODO(sbaker) migrate docker-distribution data to image-serve