engine:slave's mac in Broker, to lower case

slave's mac in older vdsm versions, is in upper case, resulting
slave's mac is saved and repressent in upper case

Change-Id: Ibfebc2aea4bc3605253bfb072fc4ad19c0fd2270
Bug-Url: https://bugzilla.redhat.com/1132422
Signed-off-by: Eliraz Levi <elevi@redhat.com>