strengthen assertion for unscoped tokens
strengthen assertion for unscoped tokens

previously, unscoped tokens with the following structure would have
passed testing:

    {"access": {"token": {"tenant": null}}}

... which wasn't the desired behavior.

Change-Id: I16a718a8936bb155d68510dcf9db8eb4d4223bf4

rodrigods/keystone,jamielennox/keystone,rajalokan/keystone,himanshu-setia/keystone,blueboxgroup/keystone,idjaw/keystone,maestro-hybrid-cloud/keystone,ilay09/keystone,promptworks/keystone,UTSA-ICS/keystone-kerberos,ilay09/keystone,mahak/keystone,vivekdhayaal/keystone,JioCloud/keystone,roopali8/keystone,reeshupatel/demo,jonnary/keystone,ajayaa/keystone,rajalokan/keystone,ilay09/keystone,vivekdhayaal/keystone,nuxeh/keystone,reeshupatel/demo,idjaw/keystone,takeshineshiro/keystone,openstack/keystone,jumpstarter-io/keystone,promptworks/keystone,openstack/keystone,JioCloud/keystone,mahak/keystone,dstanek/keystone,vivekdhayaal/keystone,himanshu-setia/keystone,ging/keystone,rodrigods/keystone,maestro-hybrid-cloud/keystone,nuxeh/keystone,jumpstarter-io/keystone,openstack/keystone,cernops/keystone,jamielennox/keystone,ajayaa/keystone,MaheshIBM/keystone,cernops/keystone,klmitch/keystone,blueboxgroup/keystone,nuxeh/keystone,roopali8/keystone,UTSA-ICS/keystone-kerberos,ging/keystone,reeshupatel/demo,dstanek/keystone,MaheshIBM/keystone,promptworks/keystone,rushiagr/keystone,klmitch/keystone,jonnary/keystone,dims/keystone,takeshineshiro/keystone,mahak/keystone,dstanek/keystone,rajalokan/keystone,dims/keystone,rushiagr/keystone,jumpstarter-io/keystone,rushiagr/keystone