Update compute attach tests to use "volume_device_name"

Some attach tests in the compute suites are using hard-coded
values for the volume device name. These should be replaced
with the compute.volume_device_name in tempest.conf.

Change-Id: I2f5da22b5f1bbd3910e4226b96e73106aa75c65a
Merge "Update compute attach tests to use "volume_device_name""