Adding the ability to set arbitrary feature gates to controller components

[omar-nahhas]

Currently, kube-aws support the ability to add arbitrary feature gates to the kubelet on the worker nodes, although those feature gates flags are not propagated to the kubelets running on the controller nor the other controller components (.i.e. apiserver, scheduler and controller manager). This PR aims to bring consistency and extend the functionality of setting feature gates to those components.

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[omar-nahhas]

/assign @davidmccormick

[codecov-io]

Codecov Report

Merging #1541 into master will increase coverage by 0.02%.
The diff coverage is 36.36%.

@@            Coverage Diff             @@
##           master    #1541      +/-   ##
==========================================
+ Coverage   25.45%   25.47%   +0.02%     
==========================================
  Files          97       97              
  Lines        5005     5016      +11     
==========================================
+ Hits         1274     1278       +4     
- Misses       3584     3591       +7     
  Partials      147      147

Impacted Files	Coverage Δ
pkg/api/cluster.go	0% <0%> (ø)	⬆️
pkg/model/node_pool_config.go	34.88% <100%> (+3.17%)	⬆️

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[davidmccormick]

Sorry @omar-nahhas, I noticed that the tests use enabled and disabled as values for configuring the feature gates. Please could you change it to either "true" or "false" so that the value corresponds to the command line argument expected by kubernetes (which uses the boolean values)? Otherwise, looks good and looking forward to this change! :)