lxc: Hoist default base image name to ContainerBucket

This is a baby step towards setting the base image name from autoserv
command line arguments.

BUG=chromium:1103904
TEST=paltry unittests

Change-Id: I0bb8ef5b4c4cf9af60a8c020ae3f7d089cb97292
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/autotest/+/2292968
Commit-Queue: Prathmesh Prabhu <[email protected]>
Tested-by: Prathmesh Prabhu <[email protected]>
Reviewed-by: Sean Abraham <[email protected]>
Reviewed-by: Jacob Kopczynski <[email protected]>
diff --git a/site_utils/lxc.py b/site_utils/lxc.py
index 4ff0b65..7062e2b 100755
--- a/site_utils/lxc.py
+++ b/site_utils/lxc.py
@@ -58,7 +58,7 @@
         utils.run('sudo true')
 
     options = parse_options()
-    image = base_image.BaseImage(container_path=options.path)
+    image = base_image.BaseImage(options.path, lxc.BASE)
     if options.setup:
         image.setup(name=options.name, force_delete=options.force_delete)
     elif options.force_delete: