Update certificate if issuer changes

[lentzi90]

What this PR does / why we need it: This makes cert-manager re-issue certificates if the issuer changes.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #813

Special notes for your reviewer:
I noted a todo here that is a bit related and may affect how this should be done.

Release note:

Update certificate if issuer changes

[jetstack-bot]

Hi @lentzi90. Thanks for your PR.

I'm waiting for a jetstack or cert-manager 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.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[kragniz]

/ok-to-test

[munnerz]

Thanks for this 😄

/lgtm
/approve

[jetstack-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lentzi90, munnerz

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