Since Pike release by default DevStack is run with all the services
as systemd unit files (USE_SCREEN=False).
Changes done in patch:
add devstack_systemd driver;
add SystemdService;
add unit tests;
update documentation.
Co-Author: Kyrylo Romanenko <kromanenko@mirantis.com>
Co-Author: Ilya Shakhat <shakhat@gmail.com>
Change-Id: I136398e3d18bafa87689a97b22a5514f4831d56e
f223fceec7