Fix more docker role logic.

roles/docker/tasks/main.yml

@@ -31,14 +31,14 @@
 - name: Fail if Docker version requested but downgrade is required
   fail:
     msg: "Docker {{ curr_docker_version.stdout }} is installed, but version {{ docker_version }} was requested."
-  when: not curr_docker_version | skipped and curr_docker_version.stdout != '' and curr_docker_version.stdout | version_compare(docker_version, '>')
+  when: not curr_docker_version | skipped and curr_docker_version.stdout != '' and docker_version is defined and curr_docker_version.stdout | version_compare(docker_version, '>')
 
 # This involves an extremely slow migration process, users should instead run the
 # Docker 1.10 upgrade playbook to accomplish this.
 - name: Error out if attempting to upgrade Docker across the 1.10 boundary
   fail:
     msg: "Cannot upgrade Docker to >= 1.10, please use the Docker upgrade playbook for this."
-  when: not curr_docker_version | skipped and curr_docker_version.stdout != '' and curr_docker_version.stdout | version_compare('1.10', '<') and docker_version | version_compare('1.10', '>=')
+  when: not curr_docker_version | skipped and curr_docker_version.stdout != '' and curr_docker_version.stdout | version_compare('1.10', '<') and docker_version is defined and docker_version | version_compare('1.10', '>=')
 
 # Make sure Docker is installed, but does not update a running version.
 # Docker upgrades are handled by a separate playbook.