$docker::service_provider is not respected by docker::run

[optiz0r]

docker::service_provider can be manually set to override the choice of service provider for the docker daemon itself, but containers created with docker::run do not respect this and use hardcoded defaults for particular operating systems. params.pp already does the work of figuring out whether the node is running systemd (which can be overridden when necessary).

I propose to add a single check to see if docker::service_provider is systemd, and if so set the necessary variables once (rather than again, duplicating those for each OS which does support systemd), and fall back to the OS-specific blocks for non-systemd OSes.

Corresponding pull request incoming.

[optiz0r]

I see I cannot actually use docker::service_provider without making significant changes to the way the code works because the defined type includes docker::params only and not the base docker class. Is this appropriate, or should the docker::run type just have a service_provider parameter which defaults to docker::params::service_provider if not set explicitly?