🏃 goimports: fix import order, add local-prefix to linter

[seanschneeweiss]

What this PR does / why we need it:
This PR sorts the imports in the order stdlib, 3rd-party, local and adds the local-prefix to the linter settings.

Which issue(s) this PR fixes
Fixes #790

Special notes for your reviewer:

TODOs:

• squashed commits

Release note:

NONE

/hold
_{Sean Schneeweiss [email protected], Daimler TSS GmbH, legal info/Impressum}

[k8s-ci-robot]

Hi @seanschneeweiss. Thanks for your PR.

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[sbueringer]

/lgtm
/ok-to-test
/test pull-cluster-api-provider-openstack-make-conformance

[jichenjc]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jichenjc, seanschneeweiss

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[jichenjc]

/hold cancel