Settings: Update defaults formats

Update the default values of the settings which pass
arguments to backends to be valid Python values and not
YAML strings.

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

mayan/apps/document_signatures/settings.py

@@ -17,9 +17,9 @@ setting_storage_backend = namespace.add_setting(
 )
 setting_storage_backend_arguments = namespace.add_setting(
     global_name='SIGNATURES_STORAGE_BACKEND_ARGUMENTS',
-    default='{{location: {}}}'.format(
-        os.path.join(settings.MEDIA_ROOT, 'document_signatures')
-    ), help_text=_(
+    default={
+        'location': os.path.join(settings.MEDIA_ROOT, 'document_signatures')
+    }, help_text=_(
         'Arguments to pass to the SIGNATURE_STORAGE_BACKEND. '
     )
 )