Derive API server LB DNS name from user-defined private DNS zone name

[dlipovetsky]

What type of PR is this?
/kind bug

What this PR does / why we need it:
If the user defines a private DNS zone name, they expect the API server LB DNS name to use that zone. Prior to this change, the API server LB DNS name always used the default private DNS zone (capz.io).

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #1885

Special notes for your reviewer:
I'm not yet familiar with the code base, so the fix and/or tests might be off-track. Constructive criticism welcome 😄

Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

The user-defined private DNS zone name is now used to derive the cluster's API server load balancer DNS name. 

[dlipovetsky]

*** Running go-apidiff ***

sigs.k8s.io/cluster-api-provider-azure/azure
  Incompatible changes:
  - GeneratePrivateFQDN: removed
+ EXIT_VALUE=1
+ set +o xtrace

I removed that function because its only argument (clusterName) is not enough to derive the FQDN, and the function is not a ClusterScope receiver, it needs to accept the private zone DNS name as an argument.

I could leave the function, but it would become (a) dead code, and (b) an incorrect implementation.

Open to suggestions!

[CecileRobertMichon]

lgtm

@dlipovetsky please squash commits!

[CecileRobertMichon]

/lgtm

[CecileRobertMichon]

/ok-to-test

[dlipovetsky]

I don't think the e2e failure is related to the change.

On a separate, and not really important note, the apidiff test passed. I guess that's because GeneratePrivateFQDN changed its argument name, but not its type. That's a false negative, but I guess the only way to make the check accurate here is to use a specific type, not a generic string, for the argument.)

[CecileRobertMichon]

/retest

[CecileRobertMichon]

/approve

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