fix multiple test failures at instance backup. #364.

backup object ID had been removed from the response
``PUT /instances/i-xxxx/backup``. But the test cases was not changed to
follow. I realized that the ID is convinent for writing test.
The response message again includes multiple backup object IDs when they
got created at the instance backup operaiton.
fix backup object state check in /backup_objects API.
change InvalidBackupObjectState error status code to 400.
add debug message to t.volume.backup.sh.
fix instance backup response.
create temp directory for each backup session.
work around checking instance backup result.
Revert "work around checking instance backup result."

This reverts commit a728a95432e7f8906a88b95ae2778ccf6a1f03da.
fix get new image uuid.
Merge pull request #369 from axsh/fix-issue-364

fix multiple test failures at instance backup.