VR: Add simple port-forwarding for the NAT4 module (DNAT)

**Description**
Add ability to statically forward ports in iptables with `-j DNAT --to-destination`.

**Use case**
Simple, static alternative to both LBs and SDNAT.

**Interface Changes**
- [x] Something like  `ONEAPP_VNF_NAT4_PORT_FWD0 = 80:1.2.3.4:8080` , similar to `ssh -L` notation maybe? (TBD)
- [ ] Add a sound interface in Sunstone