oslo.config provides parameter choices for class cfg.StrOpt to
make sure user's input must be in the choices. Option
notify_on_state_change only allow three values, so let's use
parameter choices.
UpgradeImpact: option notify_on_state_change allows three values:
'vm_state', 'vm_and_task_state', None, default value is None.
Change-Id: I94b2a8ab2584a4a31ef9644ad4f614a15636de8d