Check for dummy kernel module

[lentzi90]

What would you like to be added:
I would like to add a modprobe dummy test (or something similar) to kubernetes/preinstall.

Why is this needed:
TL;DR: Kubespray seem to run just fine without the dummy module, but the network in the cluster does not work well. Therefore I would like to quickly detect this issue while running Kubespray and alert the user.

Longer version:
I recently tried running Kubespray to install Kubernetes on some VMs based on ubuntu minimal (not completely sure it was exactly this but the template was named in this way). Kubespray run just fine and reported no issues, the cluster seemed healthy at first (ready nodes, pods running), but I quickly realized that something was wrong. Pods had trouble connecting to each other and the node-local DNS that I installed (not via Kubespray) was crash looping.
After some debugging I found that it all seemed to depend on the following:

# ip link add dummy0 type dummy
Error: Unknown device type.
# modprobe dummy
modprobe: FATAL: Module dummy not found in directory /lib/modules/5.4.0-1026-kvm

Would you be open to having such a test in the preinstall stage? If so, I would be happy to create a PR.
I tried adding the following to see if it would detect the missing module and stop, and it did so:

- name: Check dummy module
  modprobe:
    name: dummy
    state: present

Any suggestions on how to test this in a better way?

[champtar]

Can you test

- name: Check dummy module
  modprobe:
    name: dummy
    state: present
    params: 'numdummies=0'

what happens on system with already running nodelocal_dns ? I just don't want to create useless interface

[lentzi90]

It works well for detecting the missing module also with numdummies=0! I'm not too sure about how to best test these things, but I tried the following:

1. Run kubespray with the check
2. Added node-local DNS
3. Checked interfaces: only one dummy nodelocaldns
4. Rerun kubespray, still just one dummy interface
5. Remove node-local DNS and instead install it through kubespray instead. Again only the one dummy interface.
6. Rerun kubespray. Same.
7. Remove node-local DNS. The dummy interface is gone.
8. Run Kubespray. No dummy interface.

I used nmcli device status to check the interfaces.