'admin' cant be used as  username when creating VMs, Issue 958

Azure fails to provision Linux Virtual Machines when "admin" is used as username.
When this is done from the management panel the UI prevent the user to insert that username but when this happens con the CLI no meaningful errors are printed to the user. 
(It tooks me some time to figure out why some VM provisioning where failing)

So if this is the case I would strongly suggest to prevent Azure CLI to start the VM Create process (it can take many minutes) and tell the user what he is doing wrong.