Description of problem:
From the list of `flags` available via the `oc adm top` command, the `imagestreams` cannot be requested using its alias (`ìs`), as per `no` or `po` when node and pods are requested respectively.

```
$ oc adm top
Show usage statistics of resources on the server
 This command analyzes resources managed by the platform and presents current usage statistics.
Usage:
  oc adm top [flags]
Available Commands:
  images       Show usage statistics for Images
  imagestreams Show usage statistics for ImageStreams
  node         Display Resource (CPU/Memory/Storage) usage of nodes
  pod          Display Resource (CPU/Memory/Storage) usage of pods
Use "oc adm <command> --help" for more information about a given command.
Use "oc adm options" for a list of global command-line options (applies to all commands)
```


Version-Release number of selected component (if applicable):

$ oc version
Client Version: 4.6.1
Kubernetes Version: v1.19.0+d59ce34


This can be corroborated here:
```
$ oc adm top is
error: unknown command "is"
See 'oc adm top -h' for help and examples
```
```
$ oc adm top no
NAME          CPU(cores)       CPU%        MEMORY (bytes)       MEMORY%
master01      2297m            65%         5195Mi               34%
```


The aliases of `imagestreams` and `nodes` can be found here:
```
$ oc api-resources | grep "imagestreams\|nodes"
nodes                                 no                                                     false        Node
imagestreams                          is               image.openshift.io                    true         ImageStream
```


This's implemented in upstream `kubectl` (for `NewCmdTopNode`) as shown here: https://github.com/kubernetes/kubectl/blob/master/pkg/cmd/top/top_node.go#L88

A patch PR can be found here (https://github.com/openshift/oc/pull/810), but it'd need review probably.