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

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [ant31]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[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.