While it's possible to edit distro/package unit files installed to
/lib/systemd/system, a cleaner solution exists.  systemd will pick up
'drop-in' unit files located in '/etc/systemd/system/[unit name].d/'.
A drop-in unit file only has to include fields that override those in
the main unit file, and the drop-in unit and main unit file can change
independently.

This change updates the quickstart doc to use drop-in unit files to
configure docker.service instead of editing the main unit file.

See: https://coreos.com/os/docs/latest/using-systemd-drop-in-units.html

Change-Id: Iddbebf658741937e3c4157fc0a021751f10fabd2