feat(fleetctl): assume .service if not provided, like systemctl.

It also turns slashes into dashes, following systemd's criteria:
http://cgit.freedesktop.org/systemd/systemd/tree/src/shared/unit-name.c#n479
Merge pull request #300 from calavera/assume_service

feat(fleetctl): assume .service if not provided, like systemctl.