feat(vm-provision): declare the guest agent per guest OS, Linux only

Follows napalm-device-types 3.0, which replaces GUEST_AGENT_PACKAGES /
GUEST_AGENT_RUNCMD with GUEST_AGENTS = {guest_os: (packages, runcmd)}.
VMware lists Linux with open-vm-tools, as before, and refuses any other
guest_os before anything is created: BSD guests on VMware are optional
for now (NetOrk/netork#801).

The NoCloud network-config moved to napalm-device-types
(`provisioning.network_config`), which Proxmox now uses as well; the seed
module's copy is gone, and the tests check that the generic one is used.

README.md

@@ -123,7 +123,9 @@ offers for Proxmox (qcow2/raw):
 A port group fixes its VLAN, so `get_network_targets` reports each one with
 `kind="portgroup"`, `vlan_aware=False` and its `fixed_vlan_tag`; a NIC asking
 for a different `vlan_tag` is refused. The guest agent netOrk installs is
-`open-vm-tools` (`GUEST_AGENT_PACKAGES`), which reports the IP address back.
+`open-vm-tools` (`GUEST_AGENTS`), which reports the IP address back. Only
+Linux guests are provisioned; `guest_os` other than `"linux"` is refused
+(BSD guests on VMware: NetOrk/netork#801).
 
 Unverified until #305: that each distribution's cloud kernel carries the
 PVSCSI and VMXNET3 drivers.