Fix way to create login_url in dashboard test
Fix way to create login_url in dashboard test

This change can create the right login_url even if dashboard_url is
not root.

Change-Id: Icb9951aeadc08ce5c16b37c82ddb702d774256ef

bigswitch/tempest,vedujoshi/tempest,Tesora/tesora-tempest,zsoltdudas/lis-tempest,LIS/lis-tempest,izadorozhna/tempest,sebrandon1/tempest,cisco-openstack/tempest,bigswitch/tempest,Juniper/tempest,openstack/tempest,Tesora/tesora-tempest,Juniper/tempest,zsoltdudas/lis-tempest,openstack/tempest,LIS/lis-tempest,masayukig/tempest,cisco-openstack/tempest,vedujoshi/tempest,masayukig/tempest,izadorozhna/tempest,sebrandon1/tempest