NIFI-6194: Fixed verification logic to determine whether or not a var…

[markap14]

…iable's value can be changed. Previously, the logic had a bug that resulted in a failure if any child group contained a running processor that references an overridden variable and the user attempts to change the overridden variable at the higher level. We should not include any components of descendent groups if the descendent group overrides the variable.

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[bbende]

Looks good, will merge