Comment 2 for bug 1826888

@ajkavanagh you are correct that changing the rbd-image-backend while there are still instances in existence that are using a prior backend type is not supported by Nova (primarily because image conversion during migration is not supported). While it might be safer to make this option install-time-only I also think it might be a bit frustrating to not allow users to change their mind about backend type post-deployment. Therefore I think that adding a config option message (i.e. in config.yaml) to explain that this should not be changed while you still have running instances is a good idea. If we really wanted to make it one-shot then we could add an action to change it post-install.