Onboarding virtual device improvements

Onboarding with a virtual device on-premise is broken now. The command

./demo --client up

fails when the environment is already up and running, since the 'up' command now creates a user, running it again will give the error:

Creating a new user...
creating user failed: user with a given email already exists
The user already exists

The idea with the onboarding is that the user has already got the environment up and running, they are logged into the UI, and they will need a virtual device only if they don't have a physical device. In that case, they should be able to run a quick command to get a virtual device up and running.

A related note, this help text for the --client command looks outdated as 'scale' is deprecated:

-client-client Enable emulated client. To enable more than one client, you can use:   demo --client scale mender-client=2