The agent side codes need consider three scenarios:
1. Non-dvr router. The all related rules are applied in
qrouter-namespace
2. Dvr router with the local agent mode is dvr_no_external.
The all related rules are applied in snat-namespace.
3. Dvr router with the local agent mode is dvr. In this scenario,
The all related rules are applied in fip-namespace.
Change-Id: Ie8729586d318be4a673858021a0116e09e193522
Partial-Bug: #1877301