[1.10] Importing boot images on the cluster fails

The following stacktrace is from clusterd.log. This occurs when the cluster is importing images from the region.

2015-12-10 00:45:47+0000 [-] Downloading images failed.
 Traceback (most recent call last):
   File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 434, in errback
     self._startRunCallbacks(fail)
   File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 501, in _startRunCallbacks
     self._runCallbacks()
   File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 588, in _runCallbacks
     current.result = callback(current.result, *args, **kw)
   File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1184, in gotResult
     _inlineCallbacks(r, g, deferred)
 --- <exception caught here> ---
   File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1126, in _inlineCallbacks
     result = result.throwExceptionIntoGenerator(g)
   File "/usr/lib/python3/dist-packages/twisted/python/failure.py", line 389, in throwExceptionIntoGenerator
     return g.throw(self.type, self.value, self.tb)
   File "/usr/lib/python3/dist-packages/provisioningserver/pserv_services/image_download_service.py", line 123, in maybe_start_download
     yield self._start_download()
   File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1126, in _inlineCallbacks
     result = result.throwExceptionIntoGenerator(g)
   File "/usr/lib/python3/dist-packages/twisted/python/failure.py", line 389, in throwExceptionIntoGenerator
     return g.throw(self.type, self.value, self.tb)
   File "/usr/lib/python3/dist-packages/provisioningserver/pserv_services/image_download_service.py", line 114, in _start_download
     get_proxy_url("https"))
   File "/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 241, in inContext
     result = inContext.theWork()
   File "/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 257, in <lambda>
     inContext.theWork = lambda: context.call(ctx, func, *args, **kw)
   File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 118, in callWithContext
     return self.currentContext().callWithContext(ctx, func, *args, **kw)
   File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 81, in callWithContext
     return func(*args,**kw)
   File "/usr/lib/python3/dist-packages/provisioningserver/utils/twisted.py", line 197, in wrapper
     return func(*args, **kwargs)
   File "/usr/lib/python3/dist-packages/provisioningserver/rpc/boot_images.py", line 106, in _run_import
     boot_resources.import_images(sources)
   File "/usr/lib/python3/dist-packages/provisioningserver/import_images/boot_resources.py", line 269, in import_images
     sources, storage, product_mapping)
   File "/usr/lib/python3/dist-packages/provisioningserver/import_images/download_resources.py", line 286, in download_all_boot_resources
     keyring_file=source.get('keyring')),
   File "/usr/lib/python3/dist-packages/provisioningserver/import_images/download_resources.py", line 232, in download_boot_resources
     writer.sync(reader, rpath)
   File "/usr/lib/python3/dist-packages/simplestreams/mirrors/__init__.py", line 89, in sync
     return self.sync_index(reader, path, data, content)
   File "/usr/lib/python3/dist-packages/simplestreams/mirrors/__init__.py", line 236, in sync_index
     self.sync(reader, path=epath)
   File "/usr/lib/python3/dist-packages/simplestreams/mirrors/__init__.py", line 87, in sync
     return self.sync_products(reader, path, data, content)
   File "/usr/lib/python3/dist-packages/simplestreams/mirrors/__init__.py", line 323, in sync_products
     self.insert_item(item, src, target, pgree, ipath_cs)
   File "/usr/lib/python3/dist-packages/provisioningserver/import_images/download_resources.py", line 201, in insert_item
     self.store, tag, checksums, size, contentsource)
   File "/usr/lib/python3/dist-packages/provisioningserver/import_images/download_resources.py", line 122, in insert_root_image
     call_uec2roottar(root_image_path, root_tgz_path)
   File "/usr/lib/python3/dist-packages/provisioningserver/import_images/download_resources.py", line 84, in call_uec2roottar
     root_tgz_path,
   File "/usr/lib/python3/dist-packages/provisioningserver/utils/shell.py", line 129, in call_and_check
     raise ExternalProcessError(process.returncode, command, output=stderr)
 provisioningserver.utils.shell.ExternalProcessError: Command `sudo /usr/bin/uec2roottar --user=maas /var/lib/maas/boot-resources/cache/root-image-6ff7ae2389224763c785146794ee4b357dcf9e5f44597d6273282281b3262da9 /var/lib/maas/boot-resources/cache/root-tgz-6ff7ae2389224763c785146794ee4b357dcf9e5f44597d6273282281b3262da9` returned non-zero exit status 1:
 Traceback (most recent call last):
   File "/usr/bin/uec2roottar", line 11, in <module>
     from provisioningserver.import_images import uec2roottar
 ImportError: No module named provisioningserver.import_images