Enable cache_ok on custom sqlalchemy UUID type

This patch adds the "cache_ok = True" to the custom sqlalchemy UUID type in designate.
It resolves the warning:
designate/sqlalchemy/base.py:243: SAWarning: TypeDecorator UUID() 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)

Change-Id: Ic3aca3836b331db495109f6b6672e5c8af1d4028

Author: johnsom

designate/sqlalchemy/types.py

@@ -27,6 +27,7 @@ class UUID(TypeDecorator):
 
     Copied verbatim from SQLAlchemy documentation.
     """
+    cache_ok = True
     impl = CHAR
 
     def load_dialect_impl(self, dialect):