core: Update default RNG sources

This patch removes RANDOM source from "cluster required RNG sources
default" option for 3.5 clusters (this option affects default state
checkboxes in new cluster dialog in webadmin).

The reason for this change is the fact EL 6 qemu doesn't support
virtualized RNG devices, so not requiring any of RNG sources is safer.

Change-Id: I93d64231f5cc1c70164f3991c980432d40a185f6
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>
Bug-Url: https://bugzilla.redhat.com/977079