Fix bug with propagation of node caching property

[kiersten-stokes]

Fixes #3010

What changes were proposed in this pull request?

DisableNodeCaching currently requires special handling, as it is our only Elyra-owned property that is not stored and processed as an object. As a result, values weren't being stored in their instances properly and therefore weren't propagating properly. This PR adds a should_discard method to DisableNodeCaching in order to: 1.) fix the propagation issue now, and 2.) prepare for when this property is finally converted into an object-valued property in the pipeline JSON.

How was this pull request tested?

I added a case to an existing test because this has unfortunately come up before. We could potentially remove this case if desired once we switch the handling/storage of DisableNodeCaching to an object-valued properly. Instead, however, we should probably have a test case for each Elyra-owned property.

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[kevin-bates]

This looks good. My only comment is regarding the dual types that need to be handled (which is kind of a hassle) and just wondering if this is historical (and avoids a migration).

[ptitzler]

LGTM!

[kevin-bates]

LGTM