Added possibility to specify vSphere credentials via env variables

[Xartos]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespaces from that line:

/kind api-change
/kind bug
/kind cleanup
/kind design
/kind documentation
/kind failing-test
/kind feature
/kind flake

What this PR does / why we need it:

Since the vSphere password variable is in the plain text config file, it's very easy for a user to set it in plain-text while testing which might lead to unwanted consequences.

To be able to easily set sensitive credentials via other methods than from config we should be able to set it via environment variables.
This is already the case if you use --extra-vars foo=${BAR} but to make this process more straight forward I suggest we add this variable which wouldn't change the current behavior.

I used same approach as openstack and same naming convention as the official terraform module

Which issue(s) this PR fixes:

None

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

vSphere credentials can now be passed as environment variables

[k8s-ci-robot]

Hi @Xartos. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[oomichi]

The pull request message explains well, and mo part seems good for me.

/cc @oomichi
/ok-to-test

[oomichi]

Thanks for updating.

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: floryut, Xartos

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