Parameterize kubelet root directory (/var/lib/kubelet)

[ant31]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

In some situation, users wants to use a different directory than the default "/var/lib/kubelet'.
It was previously possible by setting the kubelet_custom_flags but many roles were still hardcoded to "/var/lib/kubelet".

Which issue(s) this PR fixes:

Fixes #7087

Special notes for your reviewer:

Not sure the best way to handle users that were using the kubelet_custom_flags.

Does this PR introduce a user-facing change?:

Add new variable: `kubelet_root_dir`  to change the kubelet root default "/var/lib/kubelet" to a custom directory

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ant31

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[ant31]

/retest

[ant31]

/ok-to-test

[ant31]

/hold

It's not so easy to change the default. Too many operator, csi and other software in the ecosystem have hardcoded /var/lib/kubelet.
One way to deal with this is to mount bind the new root to /var/lib/kubelet.

[ant31]

/unhold

PR is ready.
I've added a test
It's not ideal but there's no other good options AFAIK.