Following command can be used to list the kernels in centos 7
============================
# egrep ^menuentry /etc/grub2.cfg | cut -f 2 -d \' Linux Server, with Linux 3.10.0-123.el7.x86_64 Linux Server, with Linux 3.10.0-123.4.4.el7.x86_64 Linux Server, with Unbreakable Enterprise Kernel 3.8.13-35.3.2.el7uek.x86_64 Linux Server, with Unbreakable Enterprise Kernel 3.8.13-35.3.1.el7uek.x86_64 Linux Server, with Linux 0-rescue-d3e0313c0f6d48a0bb72495d2x32r1 ==================================
You can use grub2-set-default command to set default boot kernel, to set first kernel in grub2.cfg as default run and reboot.
===============
grub2-set-default 0 ===================
The information which you have provided is very good and easily understood.
If you want to boot into another kernel only once, without permanently changing the default, you can use grub2-reboot instead of grub2-set-default:
grub2-reboot 0