Fix volume name in 'test_volume_create' method

Default volume name was changed to 'ostf-test-volume'
by the commit # 00093c618ec2d013cfa964059c82c445cede068b
and this has led to permanently fail of OSTF check 'Create
volume and attach it to instance'.

Change-Id: Id9e514cc0b6ab60a4bbca33929f58afccf96bd4b
Closes-bug: #1369625