Fix service name to use on Debian depending on the precise release

Starting with Jessie, the service is now managed as "smbd" and no longer
as "samba". Fixes github issue #12.

Note: I did not use the "default" keyword parameter of filter_by to ensure
some backwards compatibility.
Merge pull request #13 from rhertzog/debian-jessie-support

Fix service name to use on Debian depending on the precise release