feature #2697: add systemd init scripts for CentOS 7

Based on openSUSE systemd scripts by Robert Schweikert

*.service files go in /lib/systemd/system
opennebula.conf goes in /lib/tmpfiles.d

sunstone, oneflow and onegate lose the *.error log files. That information
can be retrieved using systemctl status, for example for sunstone:

systemctl status -l opennebula-sunstone
feature #2697: add novnc systemd start script
feature #2697: add econe systemd script
feature #2697: update sunstone systemd to start novnc
Merge remote-tracking branch 'origin/feature-2697'