[jjo] fix panic from acmedns.go constructor failure

[jjo]

What this PR does / why we need it:
Fixes cert-manager panic.

Which issue this PR fixes : #857

Special notes for your reviewer:
FYI tested on my live install, instead of panic-ing now properly getting error instantiating acmedns challenge solver: invalid character '{' from logs.

The key fix is that last err != nil check in acme/dns/dns.go switch/case entry
for AcmeDNS, to avoid following thru and wrongly returning impl, nil (with a nil impl pointer).

Release note:

fix panic from acmedns.go constructor failure

[jjo]

/cc @Queuecumber

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[Queuecumber]

Odd that this passed all the auto testing, I'll take a look ASAP and see if I can get a PR put together

[Queuecumber]

Nevermind this is a PR, probably shouldn't look at these on my phone.

[Queuecumber]

@kragniz @munnerz Probably want to make this one a priority guys, @jjo kindly fixing my carelessness

[munnerz]

Thanks for this change @jjo!

@Queuecumber or @jjo, would you be able to add some relevant unit test cases that cover the various different scenarios/states that the config can be in? i.e. a 'valid configuration', invalid, invalid data, missing, etc? You can see some previous examples of this for other DNS providers here: https://github.com/jetstack/cert-manager/blob/master/pkg/issuer/acme/dns/dns_test.go#L73

/ok-to-test

[Queuecumber]

Yeah I was thinking last night that this should have been part of the original PR. @jjo I can take care of this if you want

[jjo]

Yeah I was thinking last night that this should have been part of the original PR. @jjo I can take care of this if you want

@Queuecumber thanks!, I'd like to take a shot at it, actually tho :)

[jjo]

@Queuecumber @munnerz: added some acmedns unittesting, PTAL thanks !

[Queuecumber]

Looks fine to me and the tests all pass, but my opinion counts for basically nothing around here so hopefully a team member can merge this soon

[munnerz]

/lgtm
/approve

[jetstack-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: munnerz

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[munnerz]

Ignore the test failures - something has been changed in our testing infra 😬

[munnerz]

/test chart-verify
/test verify

[munnerz]

/test chart-verify