Fix declaration of authentication environment variables and add test.

[richardse]

Fixes #396

Environment variables are actually a list of dictionary of the format:

env:

• name: name1
  value: value1
• name: name2
  value: value2

This PR corrects the model to read environment variables without throwing. Reference docs:

https://kubernetes.io/docs/reference/access-authn-authz/authentication/#configuration

Also adds a test.

Best reviewed in Split Mode as I've separated out a method to make it testable.

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[brendandburns]

/lgtm
/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: brendandburns, richardse

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