Be more robust about checking for reporting_db_engine presence

This matters during the test suite:
======================================================================
ERROR: Failure: AttributeError ('Namespace' object has no attribute
'reporting_db_engine')
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib/python2.6/site-packages/nose/loader.py", line 364, in
loadTestsFromName
    addr.filename, addr.module)
  File "/usr/lib/python2.6/site-packages/nose/importer.py", line 39, in
importFromPath
    return self.importFromDir(dir_path, fqname)
  File "/usr/lib/python2.6/site-packages/nose/importer.py", line 84, in
importFromDir
    mod = load_module(part_fqname, fh, filename, desc)
  File
"/d/en/fennm-0/bcfg2-dbrouter/bcfg2-dbrouter.git/testsuite/Testtools/__init__.py",
line 14, in <module>
    from common import *
  File
"/d/en/fennm-0/bcfg2-dbrouter/bcfg2-dbrouter.git/testsuite/common.py",
line 62, in <module>
    Bcfg2.DBSettings.finalize_django_config()
  File
"/d/en/fennm-0/bcfg2-dbrouter/bcfg2-dbrouter.git/src/lib/Bcfg2/DBSettings.py",
line 99, in finalize_django_config
    if opts.reporting_db_engine is not None:
AttributeError: 'Namespace' object has no attribute
'reporting_db_engine'