config:index command should not inherit from list command

[shazron]

aio-cli-plugin-config/src/commands/config/index.js

Line 15 in 6cc403b

class ConfigCommand extends ListCommand { }

This causes the issue described in adobe/aio-cli#627

It should do the same as this: https://github.com/adobe/aio-cli-plugin-app/blob/master/src/commands/app/index.js