The OAuth2.0 Access Token API is modified, support to get an OAuth2.0
certificate-bound access token from the keystone identity server with
OAuth 2.0 credentials and Mutual-TLS certificates.
Co-Authored-By: Hiromu Asahina <hiromu.asahina.az@hco.ntt.co.jp>
Change-Id: I885527bec61429b1437a046097a16491848b5a0a
Implements: blueprint support-oauth2-mtls