exact string match fails, use conventional errors.Is instead

[furkhat]

What this PR does / why we need it:

Fixes a bug when machine-controller fails on 404 when deleting already gone floating-ip or server.
Example failure

failed to delete machine at cloud provider, due to failed to clean up floating ip: failed to delete floating ip a87484bd-4ba6-4c18-8b7e-27b98606d240: Resource not found: [DELETE https://example-cloud.net:9696/v2.0/floatingips/a87484bd-4ba6-4c18-8b7e-27b98606d240], error message: {"NeutronError": {"message": "Floating IP a87484bd-4ba6-4c18-8b7e-27b98606d240 could not be found", "type": "FloatingIPNotFound", "detail": ""}}

Exact string match against "Resource not found" does not work because gophercloud v1.1.1, as well as previous versions, return

 fmt.Sprintf(
        "Resource not found: [%s %s], error message: %s",
	e.Method, e.URL, e.Body,
)

Which issue(s) this PR fixes:

Fixes #

What type of PR is this?
/kind bug

Special notes for your reviewer:

Does this PR introduce a user-facing change? Then add your Release Note here:

Fix Openstack IP cleanup

Documentation:

NONE

[kubermatic-bot]

Hi @furkhat. Thanks for your PR.

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[xrstf]

/approve

[kubermatic-bot]

LGTM label has been added.

Git tree hash: 7e79656e2b726651e9ecaba71ee832509f743f70

[xrstf]

/ok-to-test

[ahmedwaleedmalik]

/approve

[kubermatic-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahmedwaleedmalik, furkhat, xrstf

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• pkg/cloudprovider/provider/OWNERS [ahmedwaleedmalik]

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