Support advanced Nutanix network types

[embik]

Right now, our Nutanix implementation seems to assume that we use a VLAN-based network. Using a different network type (e.g. VPC overlay networking) makes the webhook fail with a nil pointer dereference. We should:

1.) Make sure that this no longer happens and that all pointers are checked correctly.
2.) Support other network types. It's not clear how complicated that is or if it's simple to implement once we figure out the incorrect assumption that the code is making.