config: Unset SCRATCH_DEV when deduced from SCRATCH_DEV_POOL

config: Unset SCRATCH_DEV when deduced from SCRATCH_DEV_POOL

In the case that we already have sections in the config file we
have to make sure that we unset SCRATCH_DEV if it has been deduced from
the SCRATCH_DEV_POOL so that it does not complain about SCRATCH_DEV in
this case.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Tested-by: Filipe David Manana <fdmanana@gmail.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>