Check for python module pyyaml when using kfctl.sh on platform GCP.

[IMBurbank]

Fixes #1739.
Stems from discussion in #1898.

Add check to kfctl.sh for platform GCP to ensure python module pyyaml is present before function updateDM calls iam_patch.py and imports yaml.

Script ends exit 1 if module not present.

Also removed a duplicate source ${ENV_FILE}.

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This change is 

[k8s-ci-robot]

Hi @IMBurbank. Thanks for your PR.

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[jlewi]

/ok-to-test

[jlewi]

This is great thank you so much.
/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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