This change adds an inherited 'privateByDefault' option,
which allows project owners to configure a project so that
all new changes will be set as private by default.
This change also removes the 'privateByDefault' config option
from gerrit.config introduced by [1] as it's not necessary any
more (enable the 'privateByDefault' in All-Project will be
inherited by all other projects).
[1] Ie7cdb6f714714
Change-Id: I6fea9f8d55c7a38ac293057681874e9b480341ed
d089b4adda