Comment 3 for bug 1826888

@hopem - yes, I think we are agreed. I don't really like config items you can't change later, so I was playing a bit of devil's advocate there :)

Essentially, to resolve this bug:

* In config.yaml of nova-compute charm: add note about issues around changing value and existing instances. Point to charms deployment guide.
* In charms deployment guide, add note about issues surrounding config value and workarounds.

Thanks.