Change the default startup choice while booting up CentOS 7.
Edit the file /etc/default/grub
Change the line
GRUB_DEFAULT="Windows 7 (loader) (on /dev/sda1)"
The value to add is determined by looking at the menuentry lines in makeactive
Choose the menuentry that you would like to be default.
To make this change active run the command
# grub2-mkconfig -o /boot/grub2/grub.cfg
You will see this change the next time you reboot.