[v0.16.0] Default Admission Controllers

[dominicgunn]

Changes

• This PR explicitly lists the admission controllers enabled by default as described [here]https://v1-16.docs.kubernetes.io/docs/reference/access-authn-authz/admission-controllers/).
• We also remove the Initializer admission controller, it hasn't existed since Kubernetes v1.10.x.

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[codecov-commenter]

Codecov Report

Merging #1857 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1857      +/-   ##
==========================================
+ Coverage   24.40%   24.42%   +0.01%     
==========================================
  Files          98       98              
  Lines        5117     5114       -3     
==========================================
  Hits         1249     1249              
+ Misses       3728     3725       -3     
  Partials      140      140              

Impacted Files	Coverage Δ
pkg/api/cluster.go	0.00% <ø> (ø)
pkg/api/types.go	0.00% <ø> (ø)

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[dominicgunn]

/lgtm

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/lgtm

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