Helper method to configure secure DFS cluster for tests

There is quite some boilerplate code for configuring a secure HDFS cluster for tests. The code is repeated in a number of test files within HBase code base. Convert the boilerplate code into a helper method to avoid duplication and lower maintenance effort.

SecureTestCluster#setHdfsSecuredConfiguration
TestSecureExport#setUpClusterKdc
TestThriftSpnegoHttpServer#addSecurityConfigurations
TestSaslFanOutOneBlockAsyncDFSOutput#setHdfsSecuredConfiguration