MB-10000: Make xdcr-cluster-name as mandatory

Change-Id: I77715e4daa96931582ac33bd1d50c39b5673f01a
Reviewed-on: http://review.couchbase.org/35975
Reviewed-by: Bin Cui <bin.cui@gmail.com>
Tested-by: Bin Cui <bin.cui@gmail.com>