Set cache_ok to avoid SAWarning

Since sqlalchemy 1.4.14[1], the following warning is raised unless
the cache_ok flag is explicitly set in classes inheriting
TypeDecorator.

SAWarning: TypeDecorator Json() will not produce a cache key because
the ``cache_ok`` attribute is not set to True.  This can have
significant performance implications including some performance
degradations in comparison to prior SQLAlchemy versions.
Set this attribute to True if this type object's state is safe to use
in a cache key, or False to disable this warning. (Background on this
error at: https://sqlalche.me/e/14/cprf)

Because the flag was set to True by default in older releases, this
adds explicit cache_ok = True, to restore the previous behavior in
Wallaby and older. (wallaby u-c has SQLAlchemy===1.3.23 while xena u-c
has SQLAlchemy===1.4.23)

[1] 6967b45020

Change-Id: I6b84a626994543468f49373a554de44025576a9a

heat/db/sqlalchemy/types.py

@@ -23,6 +23,7 @@ loads = jsonutils.loads
 class LongText(types.TypeDecorator):
 
     impl = types.Text
+    cache_ok = True
 
     def load_dialect_impl(self, dialect):
         if dialect.name == 'mysql':
@@ -45,6 +46,7 @@ class Json(LongText):
 class List(types.TypeDecorator):
 
     impl = types.Text
+    cache_ok = True
 
     def load_dialect_impl(self, dialect):
         if dialect.name == 'mysql':