Add infrastructure to document env var usage. (#12727)
Add infrastructure to document env var usage. (#12727)

- Introduce the pkg/env package containing a few functions to query environment
variable values. It keeps track of the variables requested so they can be documented.

- Extend pkg/collateral to recognize and output the environment variables used in the
process. This is what is needed to make this stuff show up on istio.io.

- Update all relevant call sites to use the new infrsstructure. It's still missing
descriptions for all the variables, that'll be up to component authors. I'll file
issues to get that work done.

- Fixed bugs in the node_agent_k8s code that was using env vars as the default for
Cobra command-line arguments, resulting in potentially variable default values
produced in the generated docs. Default values need to be static.
istio/pkg,istio/pkg,istio/pkg,istio/pkg,istio/pkg