[doc] Add a note about Ambari tests for HiveServer2 when using custom auth provider

[frbattid]

When using cosmos-hive-auth-provider Ambari tests must be modified. Such tests try to start a connection to HiveServer2 based on Hive default user and password used in the Metastore. Obviously, such credentials are not going to work with cosmos-hive-auth-provider.

Since the above test is mandatory for considering HiveServer2 started, Ambari shows a failure and does not effectively start the server. Thus, 2 approaches can be followed:

1. To modify the test, which is a Java application. Specifically, the arguments passed to the tests must include valid user and a OAuth2 token.

$ vi hive_service.py
...
db_connection_check_command = format(
        "{java64_home}/bin/java -cp {check_db_connection_jar}:/usr/share/java/{jdbc_jar_name} org.apache.ambari.server.DBConnectionVerification {hive_jdbc_connection_url} {hive_metastore_user_name} {hive_metastore_user_passwd!p} {hive_jdbc_driver}")
...

2. To remove the test at all from hive_service.py.