Verify zfs module loaded before starting services

This is a minor change to the systemd service templates that verifies
the zfs kernel module is loaded by the kernel prior to attempting to
import any zpool.

The services check for the presence of /sys/module/zfs which indicates
the zfs is module is loaded. This uses the systemd built-in check
ConditionPathIsDirectory.

Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matthew Thode <prometheanfire@gentoo.org>
Signed-off-by: Jonathon Fernyhough <jonathon.fernyhough@york.ac.uk>
Closes #10663