Fixes #6444 - add support for virtual NICs

From my point of view, managed means that foreman should configure the interface and create reservations if applicable. We can have managed interface 
* without any proxy and any external DHCP, so we configure interface statically (foreman suggests IP based on internal DB), 
* without any proxy but with external DHCP, we configure interface to use DHCP but we don't do any reservation
* with proxy configured - in this case it works as today, external DHCP should be set to false, it does not make sense otherwise

we also may want to have interface assigned to a subnet but don't manage it. Later someone could use it when we start publishing this info ENC output.