When we execute an Openstack CLI command, keystone should log in
Keystone access log that the user-agent that made the request was
'python-openstackclient' instead of the default 'python-keystoneclient'.
Therefore, when we create the authentication session we
need to send the explicit user-agent.
Closes-Bug: #1453995
Change-Id: I75087fd4bb1ff1e6f2a911bc70bf8008268276bb