honour `replication_mode' configuration option in the VMware NSX plugin

- `replication_mode' stanza determines network traffic replication mode.
  Default value for this setting is `service' which means that NSX
  cluster operates with NSX Service nodes (as any NSX production cluster
  should) and packet replication on router and switches happens on NSX
  Service nodes.
- setting `replication_mode' to `source' means that traffic replication
  happens localy on nodes, which is the only option if we do not have
  Service nodes.

DocImpact
Change-Id: I830dc687dbf25a8fd9e4d375c5e37af2bda83df4
Partial-bug: #1365043
Merge "honour `replication_mode' configuration option in the VMware NSX plugin"