Sets the default keyspace for cqlengine models when passing in an
existing session to cqlengine.connection.set_session() if the session's
keyspace is set and models.DEFAULT_KEYSPACE isn't. Without this, the
user will see an error when trying to perform Cassandra interactions
with a Model instance like "Keyspace none does not exist" unless they
explicitly set models.DEFAULT_KEYSPACE themselves ahead of time or have
a __keyspace__ attribute set on the model.
Also no longer raising an exception in cqlengine.connection.setup() if
default_keyspace isn't set since individual models might still have
__keyspace__ set.
b302778623