[IMP] split state field in two boolean fields with the same feature in b...

with the same feature in base delivery_carrier_label

validated on https://code.launchpad.net/~akretion-team/carriers-deliveries/7-split-default-option-state-from-deliv-method-dbl/+merge/224027

this PR replace the previous closed
Personally I prefer 

    if (available_option.mandatory
            or available_option.by_default):

Over

    if (available_option.mandatory or
            available_option.by_default):

First option seems more readable to me as you direclty read the or linking the two terms

So I agree that ugly \ should be removed but for position of operator unless we get a concensus I don't think we should block a PR for that.
By the way why second is it the preferred one ?