Consider hostname in determining local builds

If DIB fails to produce local files, or if we remove a failing diskimage
from the config before the ZooKeeper node has been removed, we never
remove the znode. Work around this by considering any recorded builds
that have a 'builder' that matches the host doing the cleanup as owned
by that host.

Change-Id: Id3b1e2ae0d0f71209cd66091a7d69143b0b4b20c

nodepool/builder.py

@@ -353,7 +353,9 @@ class CleanupWorker(BaseWorker):
         for build in builds:
             base = "-".join([image, build.id])
             files = DibImageFile.from_image_id(self._config.imagesdir, base)
-            if files:
+            # If we have local dib files OR if our hostname matches the
+            # recorded owner hostname, consider this our build.
+            if files or (self._hostname == build.builder):
                 ret.append(build)
         return ret