Fix test_shelve_volume_backed_instance
Fix test_shelve_volume_backed_instance

test_shelve_volume_backed_instance actually created an instance with the
volume attached as a data disk rather than the root disk. This fix makes
it create an instance with the volume as the root disk.

Change-Id: I719aa7ec2507b7936c08d17108511260830ad099
Closes-Bug: 1613946

masayukig/tempest,Juniper/tempest,vedujoshi/tempest,cisco-openstack/tempest,cisco-openstack/tempest,vedujoshi/tempest,Juniper/tempest,openstack/tempest,sebrandon1/tempest,openstack/tempest,Tesora/tesora-tempest,Tesora/tesora-tempest,masayukig/tempest,sebrandon1/tempest