Support Tacker service to obtain an OAuth 2.0 access token from an
external authorization server, and then use the access token to access
related OpenStack services that uses the external_oauth2_token filter
provided by the keystone middleware for permission authentication.
Implements: blueprint enhance-oauth2-interoperability
Change-Id: Ic6ee3c74f5a0e6b7c566033b32ae7308bc198a49