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