add support for custom config groups in cluster update

[anisf]

fixes issue #153

To test, use the following syntax in host_templates:

host_templates:
  someHostTemplateName:
    HIVE_ON_TEZ:
      - HIVESERVER2/someCustomConfigGroupName
      - GATEWAY/someCustomConfigGroupName

and the following syntax for services configs:

configs:
  HIVE_ON_TEZ:
    HIVESERVER2/someCustomConfigGroupName:
      hiveserver2_load_balancer: specific-lb.cloudera.com:7777
    GATEWAY/someCustomConfigGroupName:
      hive_client_config_safety_valve: "<property><name>hive.server2.enable.impersonation</name><value>false</value></property>"

ToDo:

• Create configuration groups
• Assign hosts to configuration groups
• Assign configuration to configuration groups