Adding version discovery

Added a new WSGI application to handle version discovery between Flask
and Pecan REST API implementations.
Added version discovery to V2 API.
Modified the version discovery of V1 API to be consistent with the one
implemented for the V2 API.
Adding testscenarios to test-requirements.txt

Change-Id: Ib69d6a1362cb10fb53bed92568db1a81a5924e51
Implements: bp/api-version-discovery