Check if Openstack application credentials are empty

[huxcrux]

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:

This PR makes it possible to use the Openstack cloud provider with username/password and not only application credentials.

Which issue(s) this PR fixes:

Fixes #7945

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[Openstack] Fix a bug where Openstack cloud provider could not be used with username/password

[k8s-ci-robot]

Hi @bl0m1. Thanks for your PR.

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[alijahnas]

/ok-to-test
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: bl0m1, floryut

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