I've found that if you delete an image in Openstack, if you then try to edit a VM which used that image you get the following error:

"No finish templates were found for this host, make sure you define at least one"

To work around this you have to assign a finish template to the operating system that is assigned to the host.

I spoke to Dominic on IRC and he said the following:

[10:31] <Dominic> thermosuk: ah, deleting the image is probably why, it doesn't differentiate between that and user_data being false. Worth filing a bug for.

So here is the bug report :)