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Allow env to be updated via specific key in resource
**What this PR does / why we need it**:
This change allows users of the `oc` client to specify a list of comma-separated keys when running `oc set env` which should be imported from a resource i.e configmap or secret
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
**Release note**:
```release-note
Allow kubectl env to specify which keys to import from a config map
```