MAAS notifications should support usernames

[Dougal Matthews]

Notifications can be created for a specific user. This functionality could be useful for admins that want to alert specific users. They can be created like this;

    maas $profile notifications create message="Hi alice" user=4

The user param needs to be the numeric user ID. However, we don't seem to expose this easily in the API (neither `users read` or `users read $username` include the ID).

It would make sense for this command to accept username, allowing the following;

    maas $profile notifications create message="Hi bob" user=bob