Fix display_name of volume for test_volumes_list

In test_volumes_list, three volumes are created for some tests.
These display_name seems wrong like "volume-%s1393726625json".
The interface name should be inserted after "volume-", but it is
appended to the tail.
This patch fixes this volume name.

Change-Id: Iee55d2405e888569455a2325fe6b2843ddf9de9d
Merge "Fix display_name of volume for test_volumes_list"