types: Set 'cache_ok'

Resolves the following warning that has been popping up on tests:

  SAWarning: TypeDecorator SoftDeleteInteger() will not produce a cache
  key because the ``cache_ok`` flag is not set to True.  Set this flag
  to True if this type object's state is safe to use in a cache key, or
  False to disable this warning.

Both SoftDeleteInteger and the various subclasses of 'JsonEncodedType'
are hashable and should return a consistent hash and key. For more
information, refer to the SQLAlchemy docs [1].

[1] https://docs.sqlalchemy.org/en/14/core/custom_types.html#sqlalchemy.types.TypeDecorator.cache_ok

Change-Id: Idf6fd858fad9521c7c5ba82c31b6d3077756abd9
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

oslo_db/sqlalchemy/types.py

@@ -21,6 +21,8 @@ class JsonEncodedType(TypeDecorator):
 
     type = None
     impl = Text
+    cache_ok = True
+    """This type is safe to cache."""
 
     def __init__(self, mysql_as_long=False, mysql_as_medium=False):
         """Initialize JSON-encoding type."""
@@ -101,6 +103,8 @@ class SoftDeleteInteger(TypeDecorator):
     """
 
     impl = Integer
+    cache_ok = True
+    """This type is safe to cache."""
 
     def process_bind_param(self, value, dialect):
         """Return the binding parameter."""