NSX: Make replication mode configurable

The replication mode on switches and routers should have been configurable
to use source replication if one did not want to deploy service node(s).
This patch fixes that by making this option configurable.

Change-Id: Id9e8043c602b5e9349c10247eab993e59db5a52c
Closes-bug: #1285383
Merge "NSX: Make replication mode configurable"