Bug 572602: Change the way that Bugzilla::Object determines what fields

Bug 572602: Change the way that Bugzilla::Object determines what fields
are required for create(). It now assumes that any column that is NOT NULL
and has not DEFAULT in the database is required. We also shift the burden
of throwing errors about empty values to the validators. This fixes the bug
that Bugzilla::Bug->create() wasn't populating default values for fields
if they weren't specified in the create() parameters.
r=timello, a=mkanat