This patch changes the way refstack-client gets keystone service id.
Now non-admin creds should be enought for that.
This patch adds support for Keystone API V3.
Keystone API version selected from tempest.conf
Section: 'identity-feature-enabled', key: 'api_v3'
Closes-Bug: #1495671
Change-Id: I1f9edffba88cf99c39b2c8b99f792088d35dbd2e