fix: fix mxnet failed to update StartTime and CompletionTime

[PeterChg]

What this PR does / why we need it:

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

[PeterChg]

/assign @johnugeorge

[coveralls]

Pull Request Test Coverage Report for Build 2782042706

• 0 of 6 (0.0%) changed or added relevant lines in 1 file are covered.
• 7 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.1%) to 39.777%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/controller.v1/mxnet/mxjob_controller.go	0	6	0.0%

Files with Coverage Reduction	New Missed Lines	%
pkg/controller.v1/mpi/mpijob_controller.go	7	78.03%

Totals
Change from base Build 2751331630:	-0.1%
Covered Lines:	2323
Relevant Lines:	5840

---

💛 - Coveralls

[terrytangyuan]

Thanks!

/lgtm
/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: PeterChg, terrytangyuan

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