Add Validation for Metadata in Thresholds

In theory, the functionality is present in terms of processing thresholds, as you can see here:

https://github.com/OpenNMS/opennms/pull/2713/files

Unfortunately, validations on thresholds.xml prevents the usage of this feature.

Here are the two major problems:

1) You cannot use the meta-data format on the value when defining thresholds through the WebUI, as it silently ignores the content as it expects a number:

https://github.com/OpenNMS/opennms/blob/opennms-26.0.0-1/opennms-webapp/src/main/java/org/opennms/web/controller/admin/thresholds/ThresholdController.java#L604

Based on the above code, you'll see an error like the following on web.log:

2020-04-29 09:02:35,084 WARN  [qtp2106500011-401] o.o.w.c.a.t.ThresholdController: Failed to parse value ('${requisition:tcpErrors|10}') as a number.

2) You cannot manually set the value with meta-data format directly on threhsolds.xml because the XSD validation fails.

<threshold description="Trigger an alert if one or more incoming packets on the whole node were not received due to an error for one measurement interval" type="high" ds-type="node" value="${requisition:tcpErrors|10}" rearm="0.0" trigger="1" filterOperator="OR" ds-name="tcpInErrors"/>

[opennms@opennms bin]$ ./config-tester thresholds.xml
...
Internal Exception: org.xml.sax.SAXParseException; lineNumber: 3; columnNumber: 292; cvc-datatype-valid.1.2.1: '${requisition:tcpErrors|10}' is not a valid value for 'double'.]

Another side effect of this is because `config-tester` fails if there is meta-data content on `threhsolds.xml` OpenNMS won't start.

In conclusion, you cannot use the meta-data feature with thresholds until the code is changed to properly handle the case when the value is not a Java Double.