Fix namespace support in kubectl aging plugin

The example plugin's comments mentioned the namespace support
limitation, awaiting kubectl to implement environment variable
propagation. Now that 1.8 is out, the requirement is satisfied.

This commit adds use of the conveyed namespace information to the
plugin example.

Fixes #55255

pkg/kubectl/plugins/examples/aging/aging.rb

@@ -22,7 +22,8 @@ class Numeric
   end
 end
 
-pods_json = `kubectl get pods -o json`
+namespace = ENV['KUBECTL_PLUGINS_CURRENT_NAMESPACE'] || 'default'
+pods_json = `kubectl --namespace #{namespace} get pods -o json`
 pods_parsed = JSON.parse(pods_json)
 
 puts "The Magnificent Aging Plugin."

pkg/kubectl/plugins/examples/aging/plugin.yaml

@@ -2,7 +2,4 @@ name: "aging"
 shortDesc: "Aging shows pods by age"
 longDesc: >
   Aging shows pods from the current namespace by age.
-  Once we have plugin support for global flags through 
-  env vars (planned for V1) we'll be able to switch
-  between namespaces using the --namespace flag.
 command: ./aging.rb