Add Javadoc, Cleanup Exit Code, Fix password generation, genericize nifi property updates

Need to add javadocs.

Exit code usage should be unified.

Password generation creates a random BigInteger base 64 encodes its byte array. This leads to biases since the BigIntegers are always positive.

Should be straightforward to switch to using the random bytes directly after base 64 encoding.

Also need to set nifi.cluster.protocol.is.secure to true, make properties updating more generic.