Provide example for setting "env" for Unified Service Tagging

[colinbjohnson]

I would like to have an example that shows how to set the "env" tag used in Unified Service Tagging.

Assuming that some kubernetes administrators have one cluster per environment this feels like a common configuration that a user of the datadog-operator would want.

The issue I'm running into is there are multiple locations where this can be set but it would be ideal to set this in one "correct" location or have an understanding of the different locations where this should be set and why.