Allow exceptions to propagate through stevedore map

Allow exceptions to propagate up through the stevedore map
function used by the V3 API servers core extension. This allows
for functionality such as extension specific parsing of client
supplied data formerly required to be handled by the core API to
be handled by the extension itself instead.

This functionality requires stevedore 0.10 or later

Partially implements blueprint nova-v3-api

Change-Id: Ib848147225707f1c7eda27b1ba796022161ba20f
Merge "Allow exceptions to propagate through stevedore map"