Revert "Use HasStandardAttributes as parent class for Tags DB model"

This reverts commit b92fc1a.

Reason for revert:
The original change was made as a “cheap win” to optimize the number
of queries the neutron server makes during testing. This did
improve the number of queries made but introduced regression in
real world deployments where some customers(through automation)
would define hundreds of tags per port across a large deployment.
I am proposing to revert this change in favor of the old “subquery”
relation in order to fix this regression. In addition, I filed the
Related-Bug #2069061 to investigate using `selectin` as the more
appropriate long term solution.

Change-Id: I437ab71f01483bb8adea96757a6d7dd6253df182
Closes-bug: #2068761
Related-Bug: #2069061

Author: mtomaska

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='joined', viewonly=True),
+        backref=orm.backref('tags', lazy='subquery', viewonly=True),
         sync_backref=False)
     revises_on_change = ('standard_attr', )