Principals in KafkaTestUtils should use canonical host name

We figured out KafkaDelegationTokenSuite is failing on AWS EC2.

During investigation we found the root cause as SPARK-39530 - from the debug message of kerberos authentication, the host part of principal seems to be required as "canonical host name".

Before SPARK-39530, the test used the IP as "127.0.0.1" (loopback) and the host part of principal as the canonical host name of "127.0.0.1".

While it is still valid to pick up the IP rather than loopback, we still need to make sure canonical host name is used in principal.