Cilium: overwrite auto-detected MTU of underlying network

[pasqualet]

What type of PR is this?
/kind feature

What this PR does / why we need it:
Overwrite the auto-detected MTU in Cilium agents.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

1. Set vagrant/config.rb:

$network_plugin = "cilium"

2. Update inventory/sample/group_vars/k8s-cluster/k8s-net-cilium.yml:

cilium_mtu: 1300

3. Run vagrant:

vagrant up

4. Verify the MTU:

for pod in $(kubectl -n kube-system get pods -o name -l k8s-app=cilium); do kubectl -n kube-system exec -it $pod -- cilium debuginfo | grep mtu ; done
mtu:1300
mtu:1300
mtu:1300

Does this PR introduce a user-facing change?:

Support the overwrite of MTU in Cilium agents

[k8s-ci-robot]

Hi @pasqualet. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[floryut]

/lgtm
/ok-to-test

[Miouge1]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Miouge1, pasqualet

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