classes: rootfs-postcommands: include /etc/fstab in overlayfs_qa_check
The systemd init manager support mount point configuration via mount
units and /etc/fstab. 'Mounts listed in /etc/fstab will be converted
into native units dynamically at boot and when the configuration of
the system manager is reloaded. In general, configuring mount points
through /etc/fstab is the preferred approach.' [1]
Read mount points from /etc/fstab to determine dynamic mount units.
