Code restructuring continuation
- Remove use of Agent::GetInstance() in interface files
- Use CfgListener::SkipNode instead of CfgListener::CanUseNode for simplicity
- Rename class PktInterface to PacketInterface
- Rename CfgFloatingIp to FloatingIpConfig, CfgServiceVlan to ServiceVlanConfig
  and CfgStaticRoute to StaticRouteConfig