Exception in CacheManager when building windows image

[dkliban]

If the /var/lib/novaimagebuilder/_cache_index does not contain any Windows artifacts and the following command is run

./nova-install --name windows-demo-image --os win2k8 --install_iso http://laforge.usersys.redhat.com/isos/winserver2008x86_64.iso --install_script autounattend.xml

The following exception is thrown:

Traceback (most recent call last):
File "./nova-install", line 147, in
sys.exit(Application().main())
File "./nova-install", line 129, in main
self.builder.run()
File "/home/stack/novaimagebuilder/novaimagebuilder/Builder.py", line 80, in run
self.os_delegate.prepare_install_instance()
File "/home/stack/novaimagebuilder/novaimagebuilder/WindowsOS.py", line 54, in prepare_install_instance
driver_locations = self.cache.retrieve_and_cache_object("driver-iso", self, None, True)
File "/home/stack/novaimagebuilder/novaimagebuilder/CacheManager.py", line 226, in retrieve_and_cache_object
self._http_download_file(source_url, local_object_filename)
File "/home/stack/novaimagebuilder/novaimagebuilder/CacheManager.py", line 295, in _http_download_file
c.setopt(c.URL, url)
TypeError: unsetopt() is not supported for this option

[dkliban]

The exception is thrown when CacheManager tries to download the driver-iso from a None url. This occurs because the current implementation assumes that the driver-iso has already been cached.