[nova] allow novnc to provide a DNS-named endpoint

[odyssey4me]

Ideally we'd like to make use of a 'name' attribute override (as with other services) in order to provide a DNS-named endpoint for the novncproxy_base_url conf entry. Overriding the URI directly affects other configurations and breaks other services, so using the 'name' method is the cleanest way to achieve this goal.