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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
```
Previously, the fake RESTClient in client-go required a Registry. It
used the Registry to fetch the GroupVersion for the fake client.
However, the way it did so was dubious in some cases (it hard-coded the
default API group in places), and not strictly necssary.
This updates the fake client to just recieve the GroupVersion and
internal group name directly, instead of requiring a Registry, so that
it can be consumed in unit tests where a Registry isn't necessarily
readily available (e.g. elsewhere in client-go).
