Determine root filesistem device and partition before running growpart

[ml0renz0]

What type of PR is this?
/kind bug

What this PR does / why we need it:
When running on Azure it determines root filesystem device and partition to run growpart instead of assuming it will always be /dev/sda1

Which issue(s) this PR fixes:

Fixes #7958

Special notes for your reviewer: We use terraform to deploy Azure environments with several disks on each VM. It often happens that a device other than /dev/sda1 is the root filesystem device on several VMs, maybe due to race conditions when deploying Azure disks and attaching them to VMs.

Does this PR introduce a user-facing change?:

Determine root filesistem device and partition before running growpart (allowing to not always be sda1)

[k8s-ci-robot]

Hi @mlorenzo-stratio. Thanks for your PR.

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[oomichi]

/ok-to-test
/lgtm

[floryut]

@mlorenzo-stratio Thanks 👍

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: floryut, mlorenzo-stratio

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