Add overhead_time_multiplier as a configurable option #615

Job	Run time
Detect Changes	12s
constantpredictor/iris	53s
constantpredictor/kc2	51s
constantpredictor/cholesterol	48s
randomforest/iris	1m 11s
randomforest/kc2	1m 21s
randomforest/cholesterol	1m 20s
autogluon/iris	1m 1s
autogluon/kc2	1m 2s
autogluon/cholesterol	1m 2s
autosklearn/iris	1m 1s
autosklearn/kc2	1m 0s
autosklearn/cholesterol	1m 2s
gama/iris	42s
gama/kc2	45s
gama/cholesterol	44s
h2oautoml/iris	1m 2s
h2oautoml/kc2	1m 0s
h2oautoml/cholesterol	1m 0s
mlplanweka/iris	1m 2s
mlplanweka/kc2	58s
mlplanweka/cholesterol	1m 0s
mlr3automl/iris	59s
mlr3automl/kc2	59s
mlr3automl/cholesterol	1m 1s
naiveautoml/iris	1s
naiveautoml/kc2	1s
naiveautoml/cholesterol	1m 1s
tpot/iris	1s
tpot/kc2	1s
tpot/cholesterol	1s
tunedrandomforest/iris	1s
tunedrandomforest/kc2	1s
tunedrandomforest/cholesterol	1s
	25m 5s

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