Add Nova v2.1 API endpoint

In Juno cycle, we started to implement Nova v2.1 API and most part
has been implemented now.
For using/testing the API, this patch adds the endpoint setting to
devstack.

Change-Id: I25557cb2b0a1384ee11d3e1ae7d424828e766e50
Merge "Add Nova v2.1 API endpoint"