core: case-insensitive name uniqueness validation on DCs and Clusters

This patch ensures no two DCs and Clusters are created with the same
name, using a case-insensitive validation (rather than the current
implementation which is case-sensitive).

Change-Id: I316efa401f742638c070561b75fc863178d8c1a8
Bug-Url: https://bugzilla.redhat.com/828080
Signed-off-by: Oved Ourfali <oourfali@redhat.com>