Not sure where this was introduced, whether systemd, grub2, or cent7 itself, but by simply adding data=writeback in to fstab for / will cause / to go ro with the message " Cannot change data mode on remount mount: / not mounted or bad option writeback"
This can be resolved by telling grub2 about the writeback option to the grub config rootflags=data=writeback
GRUB_TIMEOUT=5 GRUB_DISTRIBUTOR="$(sed 's, release .*$,,g' /etc/system-release)" GRUB_DEFAULT=saved GRUB_DISABLE_SUBMENU=true GRUB_TERMINAL_OUTPUT="console" GRUB_CMDLINE_LINUX="crashkernel=auto rhgb rootflags=data=writeback" GRUB_DISABLE_RECOVERY="true"
and recreate the grub2-config
grub2-mkconfig -o /boot/grub2/grub.cfg