Don't rely on SQLAlchemy collections magically initializing __dict__

The behavior of accessing "foo.some_collection" mutating __dict__ is going
away.   You still get an empty list back but the assumptions made
in Nova here go beyond that including that security group tests
are expecting "rules=[]" to be in `__dict__` which will not be the case.
Set an empty list explicitly here.

Related SQLAlchemy gerrit: https://gerrit.sqlalchemy.org/#/c/sqlalchemy/sqlalchemy/+/1283/
Oslo.db discussion: https://bugs.launchpad.net/oslo.db/+bug/1830504

Change-Id: I986e069a65b608799b5074829fa796c6c4b98da8

Author: zzzeek

nova/db/sqlalchemy/api.py

@@ -3861,7 +3861,7 @@ def security_group_create(context, values):
     security_group_ref = models.SecurityGroup()
     # FIXME(devcamcar): Unless I do this, rules fails with lazy load exception
     # once save() is called.  This will get cleaned up in next orm pass.
-    security_group_ref.rules
+    security_group_ref.rules = []
     security_group_ref.update(values)
     try:
         with get_context_manager(context).writer.savepoint.using(context):