Fix jsonutils.to_primitive UserWarning

The CheatingSerializer fixture used in nova tests keeps
the RequestContext.db_connection set on the context object
which otherwise wouldn't normally happen. The context object
can show up in error notification payloads because of how
nova.exception_wrapper.wrap_exception works. That payload
is eventually serialized for notifications and since the
cheated RequestContext.db_connection is set and cannot be
serialized, it results in a UserWarning from the
jsonutils.to_primitive method (called via JsonPayloadSerializer).

This will eventually result in failures when that UserWarning
is made into an error.

To fix this, we can pass a fallback method to to_primitive()
which will serialize a RequestContext object the same way that
RequestContextSerializer serializes a context - by simply
converting it to dict form.

Since this only affects test runs, because of using the
CheatingSerializer fixture, it should have no impact on
runtime serializations.

Error logging is added to the FakeNotifier since it's hard
to know what is wrong in the payload unless it is logged.

Also, the WarningsFixture is updated to make sure we don't
introduce new UserWarnings for the serialization issue.

The jsonutils.to_primitive() fallback method was added to
oslo.serialization via commit cdb2f60d26e3b65b6370f87b2e9864045651c117
in 2.21.1 so we have to bump our minimum required version
of that library as well.

Change-Id: Id9f960a0c7c8897dbb9edf53b4723154341412d6
Closes-Bug: #1799249

requirements.txt

@@ -41,7 +41,7 @@ oslo.config>=6.1.0 # Apache-2.0
 oslo.context>=2.19.2 # Apache-2.0
 oslo.log>=3.36.0 # Apache-2.0
 oslo.reports>=1.18.0 # Apache-2.0
-oslo.serialization!=2.19.1,>=2.18.0 # Apache-2.0
+oslo.serialization!=2.19.1,>=2.21.1 # Apache-2.0
 oslo.upgradecheck>=0.1.1
 oslo.utils>=3.37.0 # Apache-2.0
 oslo.db>=4.40.0 # Apache-2.0