core: Default DC & Cluster has fixed UUIDs

When installing several RHEV environments and having integrated
management for the heterogeneous virtual environment.

When the integrated management program gathering information through the
rhev REST api every RHEVM installations of The 'Default' Datacenter
uuid and 'Default' Cluster uuid are the same.

These were generated uniq uuids on previous versions implementations
such as RHEV 3.0 or 3.1.

This patch removes the static DC & Cluster UUID (except from the tests)
and change all references to take the dynamic generated value that is
stored in the vdc_options table.

Change-Id: I96c96f5692a9ba113f37f0f8b082aaee244b3ed7
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1026885
Signed-off-by: Eli Mesika <emesika@redhat.com>