fix sever not deleted issue in test_attach_detach_volume.

The server will not be deleted in test_attach_detach_volume, if
exception raise in _create_and_attach. The patch fix this issue.

bug 1153493

Change-Id: I0dea1d3dc10c623f3bae970b04b9394270005bd0
Merge "fix sever not deleted issue in test_attach_detach_volume."