RHEL 7 /CentOS 7 Boot Single User Mode

1. init=/bin/sh at the end of line that starts with linux in Grub2 (In case of  VMWare like KVM or VirtualBox use rb.break instead of inti=/bin/sh)
2. run mount -o remount,rw /sysroot when the system boots
3. run chroot /sysroot
4. run passwd
5. run touch /.autorelabel
6. run exit to leave chroot
7. run exit to logout

RHEL 7 Minimal Installation