Use headless services for Training jobs

[rc-zhang]

ref #40

cc @gaocegege

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This change is 

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[coveralls]

Coverage increased (+0.2%) to 44.647% when pulling 384fc02 on rc-zhang:headless_svc into a36b350 on kubeflow:master.

[googlebot]

CLAs look good, thanks!

[gaocegege]

Thanks for your contribution!

pkg/trainer/replicas.go:1::warning: file is not goimported (goimports)

Please fix the code style errors reported by travis :-)

[gaocegege]

/ok-to-test

[rc-zhang]

@gaocegege fixed. The issue is I didn't gofmt the file.

[gaocegege]

/lgtm

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