Add Audit Scope Handler

This patch set adds audit scope mechanism.
It also removes host_aggregate field.

Change-Id: Ia98ed180a93fc8c19599735e2b41471d322bae9a
Partially-Implements: blueprint define-the-audit-scope

watcher/common/utils.py

@@ -152,3 +152,5 @@ def extend_with_strict_schema(validator_class):
 
 StrictDefaultValidatingDraft4Validator = extend_with_default(
     extend_with_strict_schema(validators.Draft4Validator))
+
+Draft4Validator = validators.Draft4Validator