MB-9804: Provide user/password for another cluster

When adding node from one cluster to another cluster, it is required
to provide user/password for that cluster. If a node doesn't belong
to any cluster, user/password are not needed then.

Change-Id: I93009b344d0f141ba13e9dda2aac64b5338ec55f
Reviewed-on: http://review.couchbase.org/31642
Reviewed-by: Steve Yen <steve.yen@gmail.com>
Tested-by: Bin Cui <bin.cui@gmail.com>