[autotest] Refactor ContainerBucket.

Remove base container management code from ContainerBucket.  This change:

- simplifies the ContainerBucket,

- makes it clearer that the base image setup is something that is
  performed separately from normal ContainerBucket operation,

- starts the process of decoupling container creation from the
  ContainerBucket, which will be necesary to move to a container pool.

BUG=chromium:720219

TEST=sudo python base_image_unittest.py
TEST=sudo python container_bucket_unittest.py
TEST=sudo python container_unittest.py
TEST=sudo python lxc_config_unittest.py
TEST=sudo python lxc_functional_test.py
TEST=sudo python zygote_unittest.py

Change-Id: I719f7b113729c7387ddacaec77c4c63154d52ea0
Reviewed-on: https://chromium-review.googlesource.com/636033
Commit-Ready: Ben Kwa <[email protected]>
Tested-by: Ben Kwa <[email protected]>
Reviewed-by: Ben Kwa <[email protected]>
diff --git a/utils/unittest_suite.py b/utils/unittest_suite.py
index f15e94c..67ca62f 100755
--- a/utils/unittest_suite.py
+++ b/utils/unittest_suite.py
@@ -108,6 +108,7 @@
     'des_01_test.py',
     'des_02_test.py',
     # Require lxc to be installed
+    'base_image_unittest.py',
     'container_bucket_unittest.py',
     'container_unittest.py',
     'lxc_functional_test.py',