feat: upgrade control plane only

[jackfrancis]

Reason for Change:

This PR adds a --control-plane-only option to the upgrade command, to enable upgrade operations to target only the master vms.

Issue Fixed:

Fixes #2134

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• tested upgrade from previous version

Notes:

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jackfrancis

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[stale]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[codecov]

Codecov Report

Merging #2635 into master will increase coverage by 0.00%.
The diff coverage is 93.75%.

@@           Coverage Diff           @@
##           master    #2635   +/-   ##
=======================================
  Coverage   72.47%   72.48%           
=======================================
  Files         140      140           
  Lines       25683    25694   +11     
=======================================
+ Hits        18614    18624   +10     
- Misses       5993     5994    +1     
  Partials     1076     1076           

Impacted Files	Coverage Δ
cmd/upgrade.go	47.09% <50.00%> (+0.03%)	⬆️
pkg/operations/kubernetesupgrade/upgradecluster.go	82.70% <100.00%> (+0.44%)	⬆️
pkg/operations/kubernetesupgrade/upgrader.go	61.06% <100.00%> (+0.23%)	⬆️

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[mboersma]

/lgtm