NIFI-2907 Fix Flow Schema Validation Errors

[jvwing]

This PR contains two changes to flow.xml schema validation:

1.) Schema validation errors are now handled and logged to the nifi-app.log file as warnings, with text indicating that these are schema validation errors for the flow configuration. This should eliminate ominously vague nastygrams in the bootstrap log file. The new warnings look like this:

2017-03-11 12:36:13,329 WARN [main] o.a.n.c.StandardFlowSynchronizer Schema validation error parsing Flow Configuration at line 52, col 40: cvc-complex-type.2.4.a: Invalid content was found starting with element 'template'. One of '{processor, inputPort, outputPort, label, funnel, processGroup, remoteProcessGroup, connection, controllerService}' is expected.

2.) The FlowConfiguration.xsd schema has been updated to include templates. I did not detail the schema of the snippet element that forms most of a template, for the following reasons:

• I'm lazy
• While the snippet contents are logically similar to the RootProcessGroupType or ProcessGroupType, snippets are serialized through a different process which produces different element names and element ordering. I do not believe it is possible to reuse the schema types for processors, process groups, connectors, etc. A new parallel set of snippet-specific schema types would be needed, and I'm not convinced that would add value to the validation.
• Combining the two factors above, I decided that addressing the validation errors for templates was enough.

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[markap14]

@jvwing OK, got this reviewed. As far as I can tell it all follows exactly what we write out now. Many thanks for getting this addressed. Sorry it took so long to review! Got it merged to master now though. Thanks again!

[jvwing]

Thanks, @markap14