Use HasStandardAttributes as parent class for Tags DB model

Tags db model class had defined standard attribute relationship as
"subquery" which leads to massive number of SELECT sql queries to get
those standard attributes for any tag.
With this patch this db model will use relationship definition from the
HasStandardAttributes class which is the same for all models with
standard attributes and which is "joined" model.

Change-Id: If11288c89c362c8d6f97833d89b3df77e51ee363
(cherry picked from commit 85d3fff)

Author: slawqo

neutron/db/models/tag.py

@@ -26,6 +26,6 @@ class Tag(model_base.BASEV2):
     tag = sa.Column(sa.String(255), nullable=False, primary_key=True)
     standard_attr = orm.relationship(
         'StandardAttribute', load_on_pending=True,
-        backref=orm.backref('tags', lazy='subquery', viewonly=True),
+        backref=orm.backref('tags', lazy='joined', viewonly=True),
         sync_backref=False)
     revises_on_change = ('standard_attr', )