GEODE-6174: move exists method into the validator

[jinmeiliao]

Co-authored-by: Kenneth Howe khowe@pivotal.io

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[jinmeiliao]

@Petahhh @onichols-pivotal

[pdxrunner]

This provides a better division of responsibilities between the validator and mutator. Validator calls occur before the Config object is "realized" through the distributed function call to the servers. The Mutator handles persistence of the configuration after "realization".