Add model_query() to db.sqlalchemy.utils module

Added query helper - model_query() function.
There are similar functions in Nova, Cinder, and other projects, so it
makes sense to move this helper to common db code.

POC in Nova - Ic14d2214c8ce16d6b22c6f31511aa6c0838aa17a

Change-Id: I86e78eadc5a76d47452dfefbd4faef697d613b50

openstack/common/context.py

@@ -98,3 +98,14 @@ def get_context_from_function_and_args(function, args, kwargs):
             return arg
 
     return None
+
+
+def is_user_context(context):
+    """Indicates if the request context is a normal user."""
+    if not context:
+        return False
+    if context.is_admin:
+        return False
+    if not context.user_id or not context.project_id:
+        return False
+    return True