baremetal: use docker_yum_gpgkey to fetch docker GPG key

Currently, if you override docker_yum_url, the repo must contain a GPG
key at {{ docker_yum_url }}/gpg, despite the fact that the GPG key URL
can be overridden separately via docker_yum_gpgkey. This change uses
docker_yum_gpgkey consistently, avoiding the need to keep the key in the
repo.

Closes-Bug: #1934913
Change-Id: If8e6a02ce0760123f7b076c711727ef575965192

ansible/roles/baremetal/tasks/pre-install.yml

@@ -144,7 +144,7 @@
         - name: Install docker rpm gpg key
           rpm_key:
             state: present
-            key: "{{ docker_yum_url }}/gpg"
+            key: "{{ docker_yum_gpgkey }}"
           become: True
           when: docker_yum_gpgcheck | bool
       when: ansible_facts.os_family == 'RedHat'

releasenotes/notes/bug-1934913-a8d436e3d0b950b4.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Fixes an issue seen when customising the Docker Yum repository URL on
+    CentOS, where the ``docker_yum_gpgkey`` variable is not used consistently.
+    `LP#1934913 <https://bugs.launchpad.net/kolla-ansible/+bug/1934913>`__