Support configuring etcd flags

[DanielXiao]

Add Cluster variable etcdExtraArgs to set arbitrary etcd flags. Add Cluster variable tlsCipherSuites to store the default cipher suites. When etcdExtraArgs does not contain cipher-suites, the default value is set.

Add variable ETCD_EXTRA_ARGS for UX backward compatibility.

What this PR does / why we need it

We need to support configuring etcd flags since Nimbus testbeds have only low performance storages.

Which issue(s) this PR fixes

Fixes #

Describe testing done for PR

Release note

Use the variable ETCD_EXTRA_ARGS to configure etcd flags. See the following example:
ETCD_EXTRA_ARGS: "heartbeat-interval=300;election-timeout=2000"

Additional information

Special notes for your reviewer

[github-actions]

Cluster Generation A/B Results:
https://storage.googleapis.com/tkg-clustergen/4033/20221129093640/clustergen.diff.txt
Author/reviewers:
Please review to verify that the effects on the generated cluster configurations are exactly what the PR intended, and give a thumbs-up if so.

[codecov]

Codecov Report

Merging #4033 (0b0d8f2) into main (e006486) will decrease coverage by 0.79%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #4033      +/-   ##
==========================================
- Coverage   48.15%   47.35%   -0.80%     
==========================================
  Files         433      456      +23     
  Lines       43136    44595    +1459     
==========================================
+ Hits        20772    21118     +346     
- Misses      20373    21470    +1097     
- Partials     1991     2007      +16     

Impacted Files	Coverage Δ
...ons/controllers/packageinstallstatus_controller.go	77.99% <0.00%> (-1.16%)	⬇️
cmd/cli/plugin/tkr/v1alpha3/os.go	73.50% <0.00%> (-0.86%)	⬇️
cmd/cli/plugin/cluster/get_machinehealthcheck.go	11.42% <0.00%> (ø)
cmd/cli/plugin/cluster/create.go	42.22% <0.00%> (ø)
cmd/cli/plugin/cluster/credentials_update.go	8.73% <0.00%> (ø)
...in/cluster/set_machinehealthcheck_control_plane.go	21.21% <0.00%> (ø)
cmd/cli/plugin/cluster/list.go	11.36% <0.00%> (ø)
...in/cluster/get_machinehealthcheck_control_plane.go	11.11% <0.00%> (ø)
cmd/cli/plugin/cluster/available_upgrade.go	16.32% <0.00%> (ø)
cmd/cli/plugin/cluster/delete_node_pool.go	16.66% <0.00%> (ø)
... and 17 more

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

[github-actions]

Cluster Generation A/B Results:
https://storage.googleapis.com/tkg-clustergen/4033/20221130075253/clustergen.diff.txt
Author/reviewers:
Please review to verify that the effects on the generated cluster configurations are exactly what the PR intended, and give a thumbs-up if so.

[tenczar]

it seems reasonable to me. I'm becoming a little concerned with how much customization is being baked directly into every clusterclass. I think podsecuritypolicy added clusterclass customizations in an interesting way, or maybe we even adopt kustomize in the future...

[DanielXiao]

@tenczar @wjun As you can see jsonpatch is aweful, other folks can introduce a patch to change the same fields by accident and there is no way to write unit test to ensure the expected behavior. We should try runtime sdk next release so we can easily modify map, array and leverage unit tests to avoid regression.

[wjun]

LGTM