Describe the bug

Environment variables don't autocomplete within an az ... command.

To Reproduce

MY_VAR=test
echo $MY_V<tab>    # autocompletes
az vm start -n $MY_V<tab>   # does not autocomplete

Expected behavior

MY_VAR=test
az vm start -n $MY_V<tab>    # should autocomplete

Environment summary

Installed with apt on Ubuntu 17.04
az CLI version 2.0.32
GNU bash, version 4.4.19(1)-release (x86_64-pc-linux-gnu)