core: [db] moving matrialilized views function...

core: [db] moving materialized views function to dbscustom.sh

Since materialized views functionality is not common to all databases,
moving this functionality to dbcustomfunctions.sh such that it can be
overridden by empty functions with the same names

Change-Id: I696b50dc98988f91912930b5d3777add0a2ae56d
Signed-off-by: Eli Mesika <emesika@redhat.com>