set PYTHONUNBUFFERED to 1

[kuizhiqing]

What this PR does / why we need it:

As mentioned in the comment, set PYTHONUNBUFFERED to non zero value means to set python unbuffered, which is the desired state.

Checklist:

• Docs included if any changes are user facing

[coveralls]

Pull Request Test Coverage Report for Build 5521014706

• 2 of 3 (66.67%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 32.845%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/controller.v1/xgboost/xgboost.go	0	1	0.0%

Totals
Change from base Build 5520214224:	0.02%
Covered Lines:	3163
Relevant Lines:	9630

---

💛 - Coveralls

[tenzen-y]

Thanks!
/lgtm

[johnugeorge]

/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: johnugeorge, kuizhiqing

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