Populate all imageRepository fields in TKR_DATA values

[imikushin]

What this PR does / why we need it

imageRepository fields in TKR.spec.kubernetes.{etcd,pause,coredns,kube-vip} can be empty. In this case, the clients should be defaulting to TKR.spec.kubernetes.imageRepository.

This change allows to simplify ClusterClass JSON patches doing this: they can now assume imageRepository fields in TKR_DATA values are non-empty and have the correct value.

Which issue(s) this PR fixes

Fixes #4315

Describe testing done for PR

Updated unit tests to reflect the described situation.

Release note

Allow for simpler ClusterClass JSON patches accessing 
kubernetesSpec.{etcd,pause,coredns,kube-vip}.imageRepository fields in TKR_DATA values. 
They can now assume imageRepository fields in TKR_DATA values are non-empty and default to 
kubernetesSpec.imageRepository (if they are empty in the TKR).

Additional information

Special notes for your reviewer

[codecov]

Codecov Report

Merging #4298 (257317c) into main (22174df) will decrease coverage by 0.88%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #4298      +/-   ##
==========================================
- Coverage   49.42%   48.55%   -0.88%     
==========================================
  Files         452      482      +30     
  Lines       44789    46934    +2145     
==========================================
+ Hits        22139    22788     +649     
- Misses      20563    22007    +1444     
- Partials     2087     2139      +52     

Impacted Files	Coverage Δ
addons/controllers/clusterbootstrap_controller.go	64.84% <0.00%> (-1.56%)	⬇️
...ons/controllers/packageinstallstatus_controller.go	79.15% <0.00%> (ø)
cmd/cli/plugin/cluster/delete_node_pool.go	16.66% <0.00%> (ø)
cmd/cli/plugin/isolated-cluster/test/main.go	0.00% <0.00%> (ø)
cmd/cli/plugin/cluster/set_node_pool.go	14.63% <0.00%> (ø)
cmd/cli/plugin/cluster/available_upgrade.go	16.32% <0.00%> (ø)
cmd/cli/plugin/cluster/list.go	11.36% <0.00%> (ø)
cmd/cli/plugin/cluster/credentials_update.go	8.73% <0.00%> (ø)
cmd/cli/plugin/cluster/upgrade.go	58.94% <0.00%> (ø)
cmd/cli/plugin/cluster/create.go	42.22% <0.00%> (ø)
... and 28 more

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[hongtaiwang]

lgtm