Hide the command list and completion shell on the cli

Neither of these commands are intended to be called by end-users, so they are just taken-up-space in the available shell list.

This changes the cli output from:

```
-> Console/cake command_list

Welcome to CakePHP v3.0.0-dev Console
...
Available Shells:

[CORE] acl, bake, command_list, completion, i18n, server, test, upgrade
```

to:

```
-> Console/cake command_list

Welcome to CakePHP v3.0.0-dev Console
...
Available Shells:

[CORE] acl, bake, i18n, server, test, upgrade
```