Bug 256309

Upstream have patch https://github.com/canonical/cloud-init/pull/894/files

but, this add set mtu only "subnets:" section, docs and may by some dadasource (SmartOS) support only set mtu in "config:" section, linux's have any with subnets priority

`netplan.py[WARNING]: Network config: ignoring enp0s5 device-level mtu:1500 because ipv4 subnet-level mtu:1501 provided.`

proof of concept get data in config section, but linux have logic https://github.com/canonical/cloud-init/blob/master/cloudinit/net/netplan.py#L116 (line 134)

https://github.com/canonical/cloud-init/blob/master/cloudinit/net/eni.py#L107

```
--- /usr/local/lib/python3.8/site-packages/cloudinit/net/bsd.py.orig    2021-01-15 17:33:05.000000000 +0000
+++ /usr/local/lib/python3.8/site-packages/cloudinit/net/bsd.py 2021-05-31 19:30:55.808268000 +0000
@@ -37,7 +37,9 @@ class BSDRenderer(renderer.Renderer):
         ifname_by_mac = net.get_interfaces_by_mac()
         for interface in settings.iter_interfaces():
             device_name = interface.get("name")
+            device_mtu = interface.get("mtu")
             device_mac = interface.get("mac_address")
+            LOG.info('Debug interface mtu %s', device_mtu)
             if device_name and re.match(r'^lo\d+$', device_name):
                 continue
             if device_mac not in ifname_by_mac:
@@ -64,18 +66,21 @@ class BSDRenderer(renderer.Renderer):
 
             for subnet in interface.get("subnets", []):
                 if subnet.get('type') == 'static':
+                    LOG.debug('Debug mtu %s', subnet.get('mtu'))
+                    LOG.debug('Debug address %s', subnet.get('address'))
                     if not subnet.get('netmask'):
                         LOG.debug(
                             'Skipping IP %s, because there is no netmask',
                             subnet.get('address')
                         )
                         continue
-                    LOG.debug('Configuring dev %s with %s / %s', device_name,
-                              subnet.get('address'), subnet.get('netmask'))
+                    LOG.debug('Configuring dev %s with %s / %s mtu %s, device mtu %s', device_name,
+                              subnet.get('address'), subnet.get('netmask'), subnet.get('mtu'), device_mtu)
 
                     self.interface_configurations[device_name] = {
                         'address': subnet.get('address'),
                         'netmask': subnet.get('netmask'),
+                        'mtu': device_mtu,
                     }
 
     def _route_entries(self, settings, target=None):
--- /usr/local/lib/python3.8/site-packages/cloudinit/net/freebsd.py.orig        2021-01-15 17:33:05.000000000 +0000
+++ /usr/local/lib/python3.8/site-packages/cloudinit/net/freebsd.py     2021-05-31 19:31:29.486255000 +0000
@@ -19,12 +19,13 @@ class Renderer(cloudinit.net.bsd.BSDRenderer):
 
     def write_config(self):
         for device_name, v in self.interface_configurations.items():
+            net_config = 'DHCP'
             if isinstance(v, dict):
-                self.set_rc_config_value(
-                    'ifconfig_' + device_name,
-                    v.get('address') + ' netmask ' + v.get('netmask'))
-            else:
-                self.set_rc_config_value('ifconfig_' + device_name, 'DHCP')
+                net_config = v.get('address') + ' netmask ' + v.get('netmask')
+                mtu = v.get('mtu')
+                if mtu:
+                   net_config += (' mtu %d' % mtu)
+            self.set_rc_config_value('ifconfig_' + device_name, net_config)
 
     def start_services(self, run=False):
         if not run:
```

DataSourceSmartOS declare only physical section contain mtu
https://github.com/canonical/cloud-init/blob/master/cloudinit/sources/DataSourceSmartOS.py#L815