Replace reference of POWERVS_PROVIDER_ID_FORMAT with PROVIDER_ID_FORMAT

[Karthik-K-N]

/kind feature
/area provider/ibmcloud

Describe the solution you'd like
[A clear and concise description of what you want to happen.]

Power VS provider id format has been deprecated and removed via PR, PR.

We are supposed to use PROVIDER_ID_FORMAT environmental variable or --provider-id-fmt flag.

Need to update code and document to refer the same.

Anything else you would like to add:
[Miscellaneous information that will assist in solving the issue.]

[Karthik-K-N]

/good-first-issue

[k8s-ci-robot]

@Karthik-K-N:
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[kishen-v]

/assign
Will make the required changes.