Add v2.1 API router and endpoint

Now we have decided Nova v3 API interfaces are disabled and we will
change v3 interfaces to v2 ones for using v3 infrastractures as v2.1 API.
The big difference between v2 and v3 that v2 API URLs contain project-id
but v3 ones don't contain it.
This patch adds both an API router and an endpoint of v2.1 API to support
API URLs containing project-id.

This patch does not remove v3 API router and endpoint because v3 API tests
use it now. After moving all v3 code to v2.1, we can remove them.

Partially implements blueprint v2-on-v3-api

Change-Id: Iee75fbd8951c9f246f68a9c75d762335fa5c4b4b
Merge "Add v2.1 API router and endpoint"