Assuming you have already installed OpenNebula Frontend on Centos 7:
Step 1)
Copy
/usr/lib/systemd/system/opennebula.service
to
/etc/systemd/system/multi-user.target.wants/opennebula.service
Step 2)
Copy
/usr/lib/systemd/system/opennebula-sunstone.service
to
/etc/systemd/system/multi-user.target.wants/opennebula-sunstone.service
Step 3)
Reboot your server to test if it works. It should.