[baremetal]: Use $releasever in docker-ce repo

Update to CentOS 8 versions of packages in docker-ce repo (that are
now available)

Change-Id: I50d28ea31c3c29322974b91a72a2bd7999324ac7

ansible/roles/baremetal/defaults/main.yml

@@ -11,8 +11,7 @@ docker_apt_package: "docker-ce"
 
 # Docker Yum repository configuration.
 docker_yum_url: "https://download.docker.com/linux/{{ ansible_distribution | lower }}"
-# FIXME(mgoddard): $releasever rather than 7.
-docker_yum_baseurl: "{{ docker_yum_url }}/7/$basearch/stable"
+docker_yum_baseurl: "{{ docker_yum_url }}/$releasever/$basearch/stable"
 docker_yum_gpgkey: "{{ docker_yum_url }}/gpg"
 docker_yum_gpgcheck: true
 docker_yum_package: "docker-ce"