fix variable parsing when key is number

[ElvinEfendi]

What this PR does / why we need it:
As reported at #2552 turns out Nginx regex group capturing variables should be accessed by number keys rather than string keys. I changed variable parsing to take this into consideration also added test coverage for the helper function.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #2552

Special notes for your reviewer:

[ElvinEfendi]

/assign @aledbf

[aledbf]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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Needs approval from an approver in each of these files:

• OWNERS [ElvinEfendi,aledbf]

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[codecov-io]

Codecov Report

Merging #2964 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2964   +/-   ##
=======================================
  Coverage   47.58%   47.58%           
=======================================
  Files          77       77           
  Lines        5634     5634           
=======================================
  Hits         2681     2681           
  Misses       2600     2600           
  Partials      353      353

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