[core] Fix checkstyle for generator package (#915)

core/src/main/java/com/yahoo/ycsb/RandomByteIterator.java

...om/yahoo/ycsb/generator/AcknowledgedCounterGenerator.java

...va/com/yahoo/ycsb/generator/ConstantIntegerGenerator.java

.../main/java/com/yahoo/ycsb/generator/CounterGenerator.java

...main/java/com/yahoo/ycsb/generator/DiscreteGenerator.java

...n/java/com/yahoo/ycsb/generator/ExponentialGenerator.java

...src/main/java/com/yahoo/ycsb/generator/FileGenerator.java

core/src/main/java/com/yahoo/ycsb/generator/Generator.java

...ain/java/com/yahoo/ycsb/generator/HistogramGenerator.java

...ava/com/yahoo/ycsb/generator/HotspotIntegerGenerator.java

.../ycsb/generator/IncrementingPrintableStringGenerator.java

...c/main/java/com/yahoo/ycsb/generator/NumberGenerator.java

...a/com/yahoo/ycsb/generator/ScrambledZipfianGenerator.java

...in/java/com/yahoo/ycsb/generator/SequentialGenerator.java

.../java/com/yahoo/ycsb/generator/SkewedLatestGenerator.java

.../main/java/com/yahoo/ycsb/generator/UniformGenerator.java

...ava/com/yahoo/ycsb/generator/UniformIntegerGenerator.java

...com/yahoo/ycsb/generator/UnixEpochTimestampGenerator.java

.../main/java/com/yahoo/ycsb/generator/ZipfianGenerator.java

.../src/main/java/com/yahoo/ycsb/generator/package-info.java