test_live_migration: Rename test_iscsi_volume
test_live_migration: Rename test_iscsi_volume

Simply rename the test_iscsi_volume test to reflect what it is actually
testing, the live migration of instances with an attached volume.

Change-Id: I112003baee939c34d3e3a5fdadc5d9b57c9119f5

openstack/tempest,openstack/tempest