iproute spports reporting the min and max MTU of an Interface via: `ip -d link list`.
Add a wrapper to vyos.ifconfig.Interface and add checks to `verify()`
```
>>> from vyos.ifconfig import Interface
>>> tmp=Interface('eth0')
>>> tmp.get_min_mtu()
60
>>> tmp.get_max_mtu()
9000
```
This makes sense for ethernet interfaces only as all other interfaces are "virtual" and thus will be the same on all installed systems.