tests: allow setting DNSName and ChallengeKey for webhook integration tests

[jonathansp]

What this PR does / why we need it:

Custom webhooks provide a relatively easy approach to extend cert-manager acme-challenge to other DNS providers. Some in-house developed DNS providers/integrations may have restrictions, allowing just a few domains to be validated, and often example.com is not one of them.

This PR allows the base DNS name to be set using the new SetDNSName() and SetDNSChallangeKey() methods in tests only. If not set, the default value "example.com" will be used, without changing the original behavior.

NONE

[jetstack-bot]

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[jonathansp]

/assign @jakexks

[jakexks]

/approve
/lgtm
/ok-to-test

[jetstack-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jakexks, jonathansp

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[jakexks]

/kind feature
/milestone v1.3

[jakexks]

/retest

[jonathansp]

Thanks @jakexks.