NIFI-2954 Moved StandardPropertyValidator to nifi-utils, documented s…

[joewitt]

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…cope/purpose of a few util libs, removed deps from nifi-utils.

[joewitt]

Please comment on the jira discussion regarding impact to updating code. It does not impact already built items. Is not part of the public api.

[markap14]

@joewitt all looks good. I have built a new instance of NiFi and verified that all seems to work okay. I like the recent change to keep package the same. It means that only pom's have to change. I'm a +1 on the most up-to-date commit!f