Make vault-helper download URL customizable

[Wiston999]

What this PR does / why we need it: This PR lets vault-helper download URL to be modifiable to point to internal mirrors.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #393

Special notes for your reviewer:

Release note:

Allow customizable vault-helper download URL

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[simonswine]

/assign @JoshVanL
/ok-to-test

[simonswine]

I think we need documetation for that and also provide similar things for other downloads we do:

• hyperkube
• vault-helper
• wing

[Wiston999]

Hi sorry for the delay on the update.
I've added the same capability for hyperkube and also reflected those parameters on README files for both modules.

I couldn't find anything related to wing in the puppet/ subfolder so I haven't make any change related to it.

[JoshVanL]

Hi @Wiston999, thanks for the PR. Although I have also created this PR #420 that documents how to customize various URLs in puppet and terraform, I think that it might make more sense to have this as configuration options in the tarmak.yaml . This would prevent having to recompile and provide a much better UX IMO. This can be implemented much like this PR #439 but can also be propagated to terraform to set the wing URL too. Thoughts @simonswine ?

[jetstack-bot]

@Wiston999: PR needs rebase.

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