Handle default/none values in hv/be params

When a value is set to constants.VALUE_DEFAULT we have to remove it from
the specific instance dict, as this way it will be populated from the
cluster before. If instead it's specified as constants.VALUE_NONE we'll
explicitely set it to None, to override its presence with a different
values in such defaults. However, currently, we handle None values only
for hvparams, that have a real use case for them.

Reviewed-by: imsnah