dracut: 90zfs: respect zfs_force=1 on systemd systems

On systemd systems provide an environment generator in order
to respect the zfs_force=1 kernel command line option.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #11403
Closes #12195