Add pullCommand to the helm chart

[psolarcz]

What this PR does / why we need it:

Adds new variable pullCommand that allows for customisation of args command used to pull and parse openapi schema.

Which issue this PR fixes

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

• fixes #

Special notes for your reviewer:

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• [x] DCO signed
• [ ] Chart Version bumped
• [x] Variables are documented in the README.md

[banzo]

@psolarcz thank you for your contribution, can you provide an example of pullCommand in the README.md table? Also, please rebase on master so it will run the automated test.