Run zfs load-key if needed in dracut

'zfs load-key -a' will only be called if needed.  If a dataset not
needed for boot does not have its key loaded (home directories for
example) boot can still continue.

zfs:AUTO was not working via dracut, so we still need the generator
script to do its thing.

Reviewed-by: Richard Yao <ryao@gentoo.org>
Reviewed-by: Manuel Amador (Rudd-O) <rudd-o@rudd-o.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Signed-off-by: Matthew Thode <mthode@mthode.org>
Closes #6982 
Closes #7004

etc/systemd/system/zfs-import-cache.service.in

@@ -14,6 +14,7 @@ Type=oneshot
 RemainAfterExit=yes
 ExecStartPre=/sbin/modprobe zfs
 ExecStart=@sbindir@/zpool import -c @sysconfdir@/zfs/zpool.cache -aN
+ExecStartPost=/bin/bash -c "/usr/bin/systemctl set-environment BOOTFS=$(@sbindir@/zpool list -H -o bootfs)"
 
 [Install]
 WantedBy=zfs-import.target