deploy: add --karg-none argument

deploy: add --karg-none argument

If the current deployment has "rootwait root=/dev/sda2",
but the new deployment does not need "rootwait" anymore,
there is no way to clear this arg at the moment (as opposed
to "karg=root=", which overrides any earlier argument with
the same name). With "--karg-none" users can now clear all
the previous args and set new "root=":

ostree admin deploy --karg-none --karg=root=LABEL=rootfs

Closes: #1401
Approved by: cgwalters