[feature] For OVS-DPDK where iptables isn't available, add NAT rules with OpenFlow

[wangguoqin1001]

Initial implementation: Added NAT rules for OVS-DPDK using OpenFlow

Description

For OVS-DPDK environments, you can't use iptables to modify packets since they're not in kernel space. To do NAT and offer access to the Internet for the VMs, you need to do NAT inside OpenvSwitch, with OpenFlow.

In the commit are the only OpenFlow rules that I found would make NAT work. Have to manipulate the source and destination MAC addresses for both outgoing and incoming packets manually, as well as output the packets to the desired interfaces.

Branches to which this PR applies

• master

TODO List

• Need to put the names of the interfaces and the internal IP range into the configuration parameters, or use existing ones, which I'm not familiar with,
• May have some side effects in del_flows when deactivating. There shouldn't be one in theory, but I saw the rules didn't disappear after VM shutdown.