keystoneauth supports adding a user_agent info to the Session and
Adapter via app_name. Allow users to add app_name/app_name and versions as
desired. Also, add os-client-config into additional_user_agent.
As an example, once this is landed and plumbed through shade,
nodepool will set app_name='nodepool' and we'll have:
User-Agent: nodepool/0.4.0 os-client-config/1.26.1 shade/1.19.1
keystoneauth1/2.18.0 python-requests/2.13.0 CPython/2.7.12
Change-Id: I1eb4dbd2587dcbe297b5c060c3c34b68ef51ef5e
64b28d42ed