fix(cloud-templates): keep exisiting configuration after apply

[marstamm]

related to #638

[nikku]

My proposal is that we start to test element template logic against the new APIs (ElementTemplate#applyTemplate) rather than internally. That will help us long term.

[marstamm]

We are already testing against this API, just with some utility around it to ensure the templates are added beforehand:

bpmn-js-properties-panel/test/spec/provider/cloud-element-templates/cmd/ChangeElementTemplateHandler.spec.js

Lines 2273 to 2284 in 522155a

	return getBpmnJS().invoke(function(elementTemplates, elementRegistry) {
	if (isString(element)) {
	element = elementRegistry.get(element);
	}
	expect(element).to.exist;
	elementTemplates.set(templates);
	return elementTemplates.applyTemplate(element, newTemplate);
	});

[nikku]

Sorry, my bad. What I meant is "move the test to that place" (or consider not to do it).

[marstamm]

I like having it as a separate file from ElementTemplates.spec, as we are testing the behavior of the ChangeElementTemplateHandler and is easier to find in the IDE this way.

But this is just a slight preference, no strong feelings either way

[nikku]

#661 (comment)

Great, in this case let us keep it.

I'll just make sure we have some basic coverage in the element template spec, too.

[marstamm]

@nikku, how do we want to continue with this PR? From my side, the implementation for keeping configuration C8 is ready for review. Do you want to add the test cases to this PR or is this a separate issue/PR?

[marstamm]

To be confirmed: this PR solved https://camunda.slack.com/archives/C02JLRNQQ05/p1649846975972899

(double taskDefinitions after applying template to configured service task). I will add an explicit test case for this

[marstamm]

Added integration tests. This is ready for review now.

[nikku]

My appologies for not looking into this earlier. I have this PR on my list for tomorrow 🙏

[nikku]

Works as it should 👍

Found one unrelated visual glitch: #675.