Move MBeanHolder and MBeanAttribute to own files
Separate the configuration of Ganglia and JMXetric

Introduces 2 new *ConfigurationService to take care of configuring
Ganglia (GangliaXmlConfigurationService) and JMXetric
(JMXetricXmlConfigurationService).
XMLConfigurationService is now a parent class and wrapper that preserves
the current API.
This separation allows for easier testing, and thus 2 test classes have
been introduced, GangliaXmlConfigurationServiceTest and
JMXetricXmlConfigurationServiceTest.
jmxetric_test.xml is also updated to allow for the new test cases.