[fix][test] Fixes GetPartitionMetadataTest checking for linux NIC speed

[nikhil-ctds]

Motivation

This PR fixes the GetPartitionMetadataTest and GetPartitionMetadataMultiBrokerTest that is consistently failing in post commit job.

Modifications

Set LoadBalancerOverrideBrokerNicSpeedGbps configuration manually so that the test doesn't try to fetch from the Linux instance.
This test doesn't depend on NIC speed functionally.

Verifying this change

• Make sure that the change passes the CI checks.

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

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• Dependencies (does it add or upgrade a dependency): (no)
• The public API: (no)
• The schema: (no)
• The default values of configurations: (no)
• The wire protocol: (no)
• The rest endpoints: (no)
• The admin cli options: (no)
• Anything that affects deployment: (no)

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