T7390: VPP CGNAT implementation (#30)

CLI:
```
set vpp nat cgnat interface outside <interface>        # multi
set vpp nat cgnat interface inside <interface>         # multi

set vpp nat cgnat rule <rule> outside-prefix <prefix>
set vpp nat cgnat rule <rule> inside-prefix <prefix>

set vpp nat cgnat timeout udp <sec>                # default 300
set vpp nat cgnat timeout tcp-established <sec>    # default 7440
set vpp nat cgnat timeout tcp-transitory <sec>     # default 240
set vpp nat cgnat timeout icmp <sec>               # default 60
```
OP mode:
```
show vpp nat cgnat interfaces
show vpp nat cgnat mappings
show vpp nat cgnat sessions

clear vpp cgnat inside-address <address> port <port> external-address <address> port <port>
```

1 files changed, 10 insertions, 2 deletions

diff --git a/src/conf_mode/vpp_nat.py b/src/conf_mode/vpp_nat.py
index d38599388..a6bc70032 100644
--- a/src/conf_mode/vpp_nat.py
+++ b/src/conf_mode/vpp_nat.py
@@ -186,7 +186,11 @@ def verify(config):
                     raise ConfigError(
                         f'{interface} must be a VPP interface for "address-pool translation interface"'
                     )
-                iface_address = get_interface_address(interface).get('addr_info', [])[0].get('local')
+                iface_address = (
+                    get_interface_address(interface)
+                    .get('addr_info', [])[0]
+                    .get('local')
+                )
                 addresses_translation.append(iface_address)
 
         if 'twice_nat' in address_pool:
@@ -211,7 +215,11 @@ def verify(config):
                     raise ConfigError(
                         f'{interface} must be a VPP interface for "address-pool twice-nat interface"'
                     )
-                iface_address = get_interface_address(interface).get('addr_info', [])[0].get('local')
+                iface_address = (
+                    get_interface_address(interface)
+                    .get('addr_info', [])[0]
+                    .get('local')
+                )
                 addresses_twice_nat.append(iface_address)
 
     if 'static' in config: