[tempest/defcore] Configure volume device name

[tempest/defcore] Configure volume device name

Actually attach_detach_volume must be skipped because the device name
ain't reliable and that's using a hard-coded device name in config.
This test should just get the device info after the attach and use
that rather than the hard-coded config option in Tempest that ain't
likely to work - the test should have been written more dynamically.

However, set volume device name to "vdc" since "vdb" is used for config
drive when force_config_drive=True is in action.

Change-Id: Ic70de93a609df932c7749a94f71505e927df1b79
Signed-off-by: Michael Polenchuk <mpolenchuk@mirantis.com>