Inconsistent bootstrapping of nifi.properties

NiFiProperties caches the 'instance' of properties in a typical Singleton fashion.
This creates issues with tests that provide location of nifi.properties for its run when running the entire suite. Fir example when running TestStandardProcessScheduler, the first line in setup method:

System.setProperty("nifi.properties.file.path", "src/test/resources/nifi.properties");

It is absolutely meaningless when such test is ran as part of the suite