Support systemd service file

I create a script to generate systemd service file and create it when run ```make``` command.
And, I add lines to spec file to install the service file.
I'm not sure about CentOS 7 & systemd but I think we should learn how to install the service file from other packages.

e.x.)
MariaDB: https://git.centos.org/blob/rpms!postgresql/b391d7e6ab4ab00036353be40721ad22bad99385/SPECS!postgresql.spec#L1041

It seems that the macro %{_unitdir} exists to know the install directory.