simplify & enhance the node_labels.py example

[Priyankasaggu11929]

What type of PR is this?

/kind cleanup
/kind documentation

What this PR does / why we need it:

This PR enhances the existing node_labels.py example, with the following:

• make the script more generic to any kind of Kubernetes cluster, removing the hardcoded node name.
• along with previously demonstrating the use of patch_node function, it now also covers the basic concepts like listing cluster nodes using list_node() function.
• demonstrate how to get specific node object characteristics field's values, like <node_item>.metadata.name, <node_item>.metadata.labels.
• print a cleaner output (list of cluster node name & the updated labels)

❯ python3 examples/node_labels.py 
NAME		LABELS
127.0.0.1	{'beta.kubernetes.io/arch': 'amd64', 'beta.kubernetes.io/os': 'linux', 'foo': 'bar', 'kubernetes.io/arch': 'amd64', 'kubernetes.io/hostname': '127.0.0.1', 'kubernetes.io/os': 'linux'}

Which issue(s) this PR fixes:

Fixes #1434

Does this PR introduce a user-facing change?

NONE

[yliaog]

/lgtm
/approve

[Priyankasaggu11929]

/retest

[k8s-ci-robot]

@Priyankasaggu11929: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

In response to this:

/retest

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[Priyankasaggu11929]

@yliaog @roycaihw,

In order to fix the travis-ci failing jobs, I've added more commits for removing white-spaces. The new commits removed the \lgtm label. All tests are successfully passing now.

Could you please add the label again. Thank you!

[yliaog]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Priyankasaggu11929, yliaog

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