core: Installer pre-upgrade step should ensure...

Installer pre-upgrade step should ensure DB consistency using fkvalidator.sh

Fixing a problem in fkvalidator when the child table has a value that
not exists in the parent table but this value is null.
In that case fkvalidaor should not treat that data as a violation.

Change-Id: Id76d9c3e3d619ac591a334b481c8af9ca675af9e
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=893459
Signed-off-by: Eli Mesika <emesika@redhat.com>