Description of problem:
The initial Nova implementation for image signature verification introduced a single configurable of `verify_glance_signatures` to either enable or disable the feature across _all_ instance boot requests.
While a useful starting point it would be much more useful if this could be controlled per boot request. A spec was drafted in Pike to allow this but not implemented:
While this is valid request, it's been around for some time with no attached customer case. At this point, I think it's best that we close as DEFERRED. If a customer comes forward with a request for this feature, we can reopen.