kubeadm template: Combine apiserver certSANs with extraArgs

[tstromberg]

This avoids having a duplicate YAML key that overwrites one another.

Fixes #6543

[k8s-ci-robot]

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[codecov-io]

Codecov Report

Merging #6547 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #6547      +/-   ##
==========================================
+ Coverage   37.91%   37.97%   +0.05%     
==========================================
  Files         137      137              
  Lines        8683     8691       +8     
==========================================
+ Hits         3292     3300       +8     
  Misses       4978     4978              
  Partials      413      413

Impacted Files	Coverage Δ
pkg/minikube/bootstrapper/bsutil/extraconfig.go	80.82% <100%> (+1.71%)	⬆️
pkg/minikube/bootstrapper/bsutil/kubeadm.go	82.6% <100%> (+0.51%)	⬆️

[tstromberg]

/ok-to-test

[minikube-pr-bot]

All Times Minikube (PR 6547): [ 94.238360 92.501832 93.594493]
All Times minikube: [ 92.992519 93.377115 93.947979]

Average minikube: 93.439204
Average Minikube (PR 6547): 93.444895

Averages Time Per Log

+----------------------+-----------+--------------------+
|         LOG          | MINIKUBE  | MINIKUBE (PR 6547) |
+----------------------+-----------+--------------------+
| minikube v           |  0.193047 |           0.195652 |
| Creating kvm2        | 19.983851 |          19.773193 |
| Preparing Kubernetes | 49.949176 |          50.306508 |
| Pulling images       |           |                    |
| Launching Kubernetes | 21.652079 |          21.146929 |
| Waiting for cluster  |  0.060046 |           0.056750 |
+----------------------+-----------+--------------------+

[medyagh]

Looks good, thanks for fixing it. this was a mind consuming one for me when I was refactoring for kic