contrib: systemd: make socket-activation Restart=always

Do as was done to f09a78cd219b24d4308034c8dd13410cfe5fbec7 in the
socket-activation example.

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)
Merge pull request #3834 from philips/fix-socket-activation-systemd

contrib: systemd: make socket-activation Restart same as the main service file