Use a community hosted image in MXJob E2E

[tenzen-y]

What this PR does / why we need it:
I replaced the MXJob E2E image with a community-hosted image. Also, I fixed the bug in a MXJob example.

Which issue(s) this PR fixes (optional, in Fixes #<issue number>, #<issue number>, ... format, will close the issue(s) when PR gets merged):
Fixes #

Checklist:

• Docs included if any changes are user facing

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: tenzen-y

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• OWNERS [tenzen-y]

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[coveralls]

Pull Request Test Coverage Report for Build 6423232289

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 10 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.05%) to 42.802%

---

Files with Coverage Reduction	New Missed Lines	%
pkg/controller.v1/pytorch/pytorchjob_controller.go	4	67.42%
pkg/controller.v1/xgboost/xgboostjob_controller.go	6	60.39%

Totals
Change from base Build 6418619650:	-0.05%
Covered Lines:	3746
Relevant Lines:	8752

---

💛 - Coveralls

[tenzen-y]

/assign @johnugeorge @terrytangyuan @kuizhiqing

[terrytangyuan]

/lgtm