Re-add /etc/systemd/system, mistakenly removed. Blacklist /etc/fstab
Re-add /etc/systemd/system, mistakenly removed. Blacklist /etc/fstab

The previous commit accidentally removed /etc/systemd/system from
the SYSTEM_DIRS list. We need to blacklist /etc/fstab, otherwise
systemd will auto-generated lots of XXX.mount units that don't
apply inside the container

agx/libvirt-sandbox-debian,erenyagdiran/libvirt-sandbox,libvirt/libvirt-sandbox,gczarcinski/libvirt-sandbox,gczarcinski/libvirt-sandbox,gczarcinski/libvirt-sandbox,erenyagdiran/libvirt-sandbox,erenyagdiran/libvirt-sandbox,libvirt/libvirt-sandbox,kraman/libvirt-sandbox,agx/libvirt-sandbox-debian,gczarcinski/libvirt-sandbox,kraman/libvirt-sandbox,kraman/libvirt-sandbox,agx/libvirt-sandbox-debian,gczarcinski/libvirt-sandbox,libvirt/libvirt-sandbox,agx/libvirt-sandbox-debian,erenyagdiran/libvirt-sandbox,kraman/libvirt-sandbox,agx/libvirt-sandbox-debian,kraman/libvirt-sandbox,gczarcinski/libvirt-sandbox,kraman/libvirt-sandbox