backward compatibility for k8s version < 1.14

[johnlanni]

What this PR does / why we need it:
It fixes a bug of Admission Validating Webhook not working when k8s version < 1.14

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #4468

Special notes for your reviewer:

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Hi @johnlanni. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[johnlanni]

I signed it

[johnlanni]

I signed it

[aledbf]

/ok-to-test

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@1c42362). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #4521   +/-   ##
=========================================
  Coverage          ?   59.19%           
=========================================
  Files             ?       89           
  Lines             ?     6781           
  Branches          ?        0           
=========================================
  Hits              ?     4014           
  Misses            ?     2334           
  Partials          ?      433

Impacted Files	Coverage Δ
internal/admission/controller/main.go	83.78% <100%> (ø)

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[aledbf]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, johnlanni

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[aledbf]

/retest

[aledbf]

/test pull-ingress-nginx-e2e-1-13

[aledbf]

/test pull-ingress-nginx-e2e-1-13