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
