[nova] boot from volume with root_gb=0 flavors is not allowed now

Hi everyone,

Just wanted to give a heads up about the change that recently merged in Nova which will affect Train (and current master, for deployment projects).

https://review.openstack.org/#/c/603910/

As of that change, Nova will no longer allow you to to create a VM when the root_gb of a flavor is set to 0. This is to avoid a reported security issue which we've found a while ago: https://launchpad.net/bugs/1739646

If you're using devstack, you'll probably be okay as the following change has already fixed it:

https://review.openstack.org/#/c/619319/

However, deployment projects will need to make that adjustment as well as deployers need to plan accordingly (everyone is going to upgrade to Train the day we release, right?)