Improve volume_backed logic in create_test_server

common method create_test_server can boot server
from volume if requested with arg volume_backed but
there are few issues in that:
- It takes 'delete_vol_on_termination' as arg to decide auto
  created volume should be deleted on server termination but
  if it is being passed false then created volume will be leaked
- Volume_backed logic needs to pass right name field to service
  client as per API version client it use.
- Improve the doc string for volume_backed as it is auto volume
  boot things and should be clear about default bdm inputs it use.

This commit fix above issues.

Change-Id: I6572e939aea7121a3b2efa25f65d558030223751
1 file changed