Bug Fix - support environment variable for TF_VAR_private_key and TF_VAR_private_key_password

[jeremy-albinet]

When a Default value is defined, the DefaultFunc is ignored/not called. Use the second parameter of DefaultFunc to put the default value.

To reproduce the issue, try to do a plan using TF_VAR_private_key or OCI_PRIVATE_KEY instead of TF_VAR_private_key_path

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[jeremy-albinet]

@jotruon Any chance to have review for this one?
Thanks in advance

[jotruon]

assign reviewer and testing for this PR

[jeremy-albinet]

assign reviewer and testing for this PR

I don't think I'm allowed to do it or I didn't find how to do it (no button under/near reviewers)

[tf-oci-pub]

Thank you for your valuable contribution. We greatly appreciate your efforts in submitting this pull request. However, I regret to inform you that we are unable to merge it directly on GitHub at this time.

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