HDDS-15621. TestSecureOzoneCluster fails locally

[adoroszlai]

What changes were proposed in this pull request?

TestSecureOzoneCluster fails locally (also reproducible in flaky-test-check):

Expecting actual:
  "scm-sub@localhost"
to contain:
  "<hostname>" 
	at org.apache.hadoop.ozone.TestSecureOzoneCluster.validateCertificate(TestSecureOzoneCluster.java:1360)
	at org.apache.hadoop.ozone.TestSecureOzoneCluster.testSecureOmReInit(TestSecureOzoneCluster.java:883)

because SCM is configured to localhost:

ozone/hadoop-ozone/integration-test/src/test/java/org/apache/hadoop/ozone/TestSecureOzoneCluster.java

Line 213 in f9552d0

conf.set(OZONE_SCM_CLIENT_ADDRESS_KEY, "localhost");

but the test expects the actual hostname in certificate issuer name.

Also improve assertions by replacing assertTrue with assertThat.

https://issues.apache.org/jira/browse/HDDS-15621

How was this patch tested?

$ mvn -am -pl :ozone-integration-test -Dtest='TestSecureOzoneCluster' -DskipRecon clean test
...
Tests run: 16, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 232.2 s -- in org.apache.hadoop.ozone.TestSecureOzoneCluster
...

[chungen0126]

Overall, looks good to me. Just a nit.