Replace localhost with node_name env var to access k8s proxy

[MichaelKatsoulis]

What does this PR do?

Replaces the default host for proxy data stream in kubernetes package from localhost:10249 to ${env.NODE_NAME}:10249.
The proxy metrics can be fetched from the pod by using the k8s node name and proxy port as it is accessible from inside the pod.
This is a step towards getting rid of hostNetwork:true setting in agent deployment configuration.
elastic/beats#25739

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• If I'm introducing a new feature, I have modified the Kibana version constraint in my package's manifest.yml file to point to the latest Elastic stack release (e.g. ^7.13.0).

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• Start Time: 2021-09-01T09:38:49.670+0000

• Duration: 29 min 25 sec

• Commit: ea86f2a

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Total	113

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[cfloressuazo]

How should I setup by default the kubernetes integration with fleet? It seems like this PR breaks some of the ELK manifests for monitoring kubernetes