Add validation for evaluator

[DeliangFan]

Add validation for evaluator so that the number of evaluator
should no more than 1.

Close #894

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This change is 

[k8s-ci-robot]

Hi @DeliangFan. Thanks for your PR.

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[DeliangFan]

/assign @johnugeorge

[coveralls]

Coverage increased (+0.2%) to 65.011% when pulling c10b155 on DeliangFan:validation-for-evaluator into 2fde284 on kubeflow:master.

[johnugeorge]

Can you add one unit test in validation_test.go ?

[johnugeorge]

/ok-to-test

[gaocegege]

/retest

[gaocegege]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: gaocegege

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[DeliangFan]

This fix #894

[gaocegege]

Thanks for your contribution! I think the test should be passed. Let's wait.