Make docker::run use $service_name instead of "docker"

[benningm]

When using the module with 'docker-latest' packages from RedHat the systemd service of docker is also 'docker-latest'. There is already a $service_name variable for this, but 'docker' is still hardcoded in some strings.

This replaces hardcoded 'docker' strings with $service_name.

[gdlx]

Your PR is missing service overrides template: https://github.com/garethr/garethr-docker/blob/master/templates/etc/systemd/system/docker.service.d/service-overrides-rhel.conf.erb#L2

[benningm]

Just added the service overrides and resynced with upstream