Fix AttributeError on BadRequest in scenario tests

Fixing the following incorrectly raised exception:

_StringException: Traceback (most recent call last):
  File "tempest/scenario/manager.py", line 251, in tearDownClass
    if e.__class__.__name__ == 'NotFound' or e.status_code == 404:
AttributeError: 'BadRequest' object has no attribute 'status_code'

Change-Id: I3ee006720328c0a5aabbc7766f997a032dfa8230
Merge "Fix AttributeError on BadRequest in scenario tests"