Add conf to replace nova V2 endpoint with V2.1 API

Now Nova contains both v2 and v2.1 APIs as its REST API, and we are
testing v2 API only on the gate. To test v2.1 API also, this patch
add a new item DEVSTACK_GATE_NOVA_REPLACE_V2_ENDPOINT_WITH_V21_API
which allows us to switch a compute endpoint. The default value is
for testing v2 API and we will be able to test v2.1 API by setting
1 as the value.

Devstack patch for adding same configuration in devstack-
Iaaa530bda6bcdae75e86be8dbe572abe4396e8e9

Change-Id: I29e2245e5b2be9bf57ba74a0fe00f335d1b170ec
Merge "Add conf to replace nova V2 endpoint with V2.1 API"