Update migrations to accept migrated settings

- Unify all conditional YAML load under utils.smart_yaml_load.
- Update all setting migrations to use the smart setting test
  mixin to create the test config file.

Signed-off-by: Roberto Rosario <roberto.rosario@mayan-edms.com>

mayan/apps/documents/setting_migrations.py

@@ -1,7 +1,7 @@
 from __future__ import unicode_literals
 
-from mayan.apps.common.serialization import yaml_load
 from mayan.apps.smart_settings.classes import NamespaceMigration
+from mayan.apps.smart_settings.utils import smart_yaml_load
 
 
 class DocumentsSettingMigration(NamespaceMigration):
@@ -10,11 +10,7 @@ class DocumentsSettingMigration(NamespaceMigration):
     but YAML valid too. Changed in version 3.3.
     """
     def documents_cache_storage_backend_arguments_0001(self, value):
-        return yaml_load(
-            stream=value or '{}',
-        )
+        return smart_yaml_load(value=value)
 
     def documents_storage_backend_arguments_0001(self, value):
-        return yaml_load(
-            stream=value or '{}',
-        )
+        return smart_yaml_load(value=value)