Initialize glance image directory with default value

This patch sets the deafult value of glance image directory
config option 'images_directory' from 'None' to '$state_path/images'.

Closes-Bug: #1643127
Change-Id: I4f7792dc74f36e626b00e000b227b0967ba43700

zun/conf/image_driver.py

@@ -13,6 +13,8 @@
 
 from oslo_config import cfg
 
+from zun.conf import path
+
 image_driver_opts = [
     cfg.ListOpt(
         'image_driver_list',
@@ -31,7 +33,7 @@ Interdependencies to other options:
 glance_driver_opts = [
     cfg.StrOpt(
         'images_directory',
-        default=None,
+        default=path.state_path_def('images'),
         help='Shared directory where glance images located. If '
              'specified, docker will try to load the image from '
              'the shared directory by image ID.'),

zun/conf/path.py

@@ -33,6 +33,11 @@ path_opts = [
 ]
 
 
+def state_path_def(*args):
+    """Return an uninterpolated path relative to $state_path."""
+    return os.path.join('$state_path', *args)
+
+
 def register_opts(conf):
     conf.register_opts(path_opts)