Fix bug 1022739.
To get OUI per cluster is not realistic. So This commit
make 4th octet of mac_range configurable. By this commit,
the cloud provider can allocate OUI (3 octets) + 1 extra octet
range (4 octets) for each cluster.
Change-Id: Ibbd27e6e7f16d73dfd3045ed60f63a38b23ea1ed