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Allow env from resource with keys & updated tests

**What this PR does / why we need it**:
This change allows users to pull environment from specific keys in secrets and configmaps using the `kubectl set env` command. User can provide a list of comma-separated keys with the `--keys` flag.

This can be useful when a number of applications want to share a configuration object but don't want to pollute a resource with unused environment

Improves test coverage of set env command

**Release note**:

```
Allow kubectl set env to specify which keys to import from a resource
```