VLAN configuration for --network_(device|config) should be reworked

[ydirson]

While working on LACP bonding support, I realize that VLAN configuration on cmdline allows some non-sensical configurations like:

--network_device=all --network_config=dhcp:vlan=10

Also "just specifying a VLAN on a given interface" requires today to repeat the dhcp default with no good reason:

--network_device=eth0 --network_config=dhcp:vlan=10

I think we should move the VLAN specification to the --network_device option, like

--network_device=eth[:vlan=VLAN]|mac[:vlan=VLAN]|all

The "just specifying a VLAN on a given interface" usecase will then become:

--network_device=eth0:vlan=10