Support optional on env valueFrom for secret key/configmap key

[mbarrien]

Description

Adds ability for secret key and config map key refs used in value_from for environment variables to be optional. Fixes documentation typos. Adds structures_container unit tests.

Picks back up where #295 left off.

Acceptance tests

• Have you added an acceptance test for the functionality being added?
• Have you run the acceptance tests on this branch? (If so, please include the test log in a gist)

References

Closes #240. Closes #531.

==> Checking that code complies with gofmt requirements...
go test "./kubernetes" || exit 1
ok  	github.com/terraform-providers/terraform-provider-kubernetes/kubernetes	5.776s
echo "./kubernetes" | \
		xargs -t -n4 go test  -timeout=30s -parallel=4
go test -timeout=30s -parallel=4 ./kubernetes
ok  	github.com/terraform-providers/terraform-provider-kubernetes/kubernetes	3.231s

[dak1n1]

I ran the acceptance tests on this, reviewed the docs and compared against the Kubernetes API docs, and compiled to try the new code out manually with a couple scenarios. It works great! Thanks for your work on this. The tests are a really nice addition too.

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

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