fix: kernel start failed when kubeconfig is unset

[minshenglin]

What this PR does / why we need it:

add KUBERNETES_SERVICE_HOST and KUBERNETES_SERVICE_PORT to env whitelist so kubeflow kernel can inherit those variables from kernel gateway.

Which issue(s) this PR is related to (optional, link to 3rd issue(s)):

Fixes #81

Special notes for your reviewer:

/cc @your-reviewer

Release note:

NONE

[coveralls]

Pull Request Test Coverage Report for Build 73

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 87.5%

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Totals
Change from base Build 72:	0.0%
Covered Lines:	357
Relevant Lines:	408

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💛 - Coveralls

[gaocegege]

/lgtm /approve

Thanks for your contribution! :tada: :+1: It is a known bug in the kernel

[caicloud-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: gaocegege

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