Adds API version discovery support for V3

Adds version information for the V3 API which is only displayed
when the V3 API is enabled. Even if the the V3 API is enabled the
V3 API status is "EXPERIMENTAL" and the V2 one "CURRENT". This was
done so autodiscovery tools would not yet use the V3 version by
default.

Ports the relevant parts of the version extension and associated
tests to the V3 API to display V3 version information for /v3 GET
requests.

DocImpact

Partially implements blueprint nova-v3-api

Change-Id: Idd335ce0df63d91e94a4a757f1fbae94b576c37e
Merge "Adds API version discovery support for V3"