MDL-28488 backup - conditionaly apply defaults based on mode

MDL-28488 backup - conditionaly apply defaults based on mode

Some modes of backup cannot be preloaded with general backup
settings because they may conflict with the definitions required/set
by the mode (for example, import backups are, by definition, without
users always, so we cannot apply one default in the opposite direction).

So, after the patch, current general_backup setting will be only applied
to general backups, import/hub modes won't be using any default and
automated backup defaults should be loaded here (TODO)