rest: enable Keystone auth_token middleware by default

This changes enable the Keystone middleware by default so we are able to
authenticate users. The tests are updated to transparently send an
authentication token.

We rely on WebTest>=2.0.16 in order to be able to override the
do_request() method of the WebTest application

Change-Id: Ib4cda0db388f88fb5c8aac5a092ef58028c1d301
Merge "rest: enable Keystone auth_token middleware by default"