Fix for Bug NMS-5219

Threshold expressions for node-level metrics doesn't work on non SNMP
Collectors.

The code for the XmlCollector, JdbcCollector and WmiCollector has been
updated in order to ensure that only one node-level resource is going to
be created per collectionSet.

Note: the HttpCollector must be updated as well, but considering the
complexity of the current implementation, I've decided to leave it
untouched. Avoid creating expression based thresholds for this
collector.
Merge remote-tracking branch 'origin/1.12'

Conflicts:
	features/jdbc-collector/src/main/java/org/opennms/netmgt/collectd/jdbc/JdbcCollectionSet.java
	opennms-wmi/src/main/java/org/opennms/netmgt/collectd/wmi/WmiCollectionSet.java
	protocols/xml/src/main/java/org/opennms/protocols/xml/collector/AbstractXmlCollectionHandler.java
	protocols/xml/src/main/java/org/opennms/protocols/xml/collector/XmlCollectionSet.java