When you install Centos 7 (or Centos 6), there is a link in the main configuration screen for networking. You have to click on that screen and then turn on networking in the top right of the resulting screen. That will install centos with networking enabled. Likely whoever installed your system didn't do that.
In your situation since you are using 'service' rather than 'systemctl', I guess you are running Centos 6 or earlier. I think running
chkconfig network on
Will make networking start on boot.
Your solution does not work
This is what I have in my network and network-scripts/ifcfg-eth0 files for a Centos 6 machine on which networking successfully starts at boot. My configuration is a little diffferent than yours, in that I don't use dhcp:
[root@developmentServer1 sysconfig]# cat network
[root@developmentServer1 sysconfig]# cat network-scripts/ifcfg-eth0
I do not even have a defined service named "NetworkManager":
[root@developmentServer1 sysconfig]# chkconfig --list | grep -i network
network 0:off 1:off 2:on 3:on 4:on 5:on 6:off
[root@developmentServer1 sysconfig]# cat /etc/redhat-release
CentOS release 6.4 (Final)
If you do, the suggestion to disable it is a worthwhile one to try.