[Cherry-pick] Support job scale down to zero

[hzxuzhonghu]

Automated cherry pick of #945 and #964 onto release 0.4

[TravisBuddy]

Travis tests have failed

Hey @hzxuzhonghu,
Please read the following log in order to understand the failure reason.
It'll be awesome if you fix what's wrong and commit the changes.

TravisBuddy Request Identifier: a0fca950-d09c-11ea-b8d2-e37d5f97d09d

[TravisBuddy]

Hey @hzxuzhonghu,
Something went wrong with the build.

TravisCI finished with status errored, which means the build failed because of something unrelated to the tests, such as a problem with a dependency or the build process itself.

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TravisBuddy Request Identifier: 3d519210-d0b7-11ea-b8d2-e37d5f97d09d

[k82cn]

/approve

[volcano-sh-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hzxuzhonghu, k82cn

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[TravisBuddy]

Hey @hzxuzhonghu,
Something went wrong with the build.

TravisCI finished with status errored, which means the build failed because of something unrelated to the tests, such as a problem with a dependency or the build process itself.

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TravisBuddy Request Identifier: fc4504c0-d172-11ea-bb2f-e1a2ee6530fc

[TravisBuddy]

Hey @hzxuzhonghu,
Something went wrong with the build.

TravisCI finished with status errored, which means the build failed because of something unrelated to the tests, such as a problem with a dependency or the build process itself.

View build log

TravisBuddy Request Identifier: d051a100-d179-11ea-bb2f-e1a2ee6530fc

[TravisBuddy]

Hey @hzxuzhonghu,
Something went wrong with the build.

TravisCI finished with status errored, which means the build failed because of something unrelated to the tests, such as a problem with a dependency or the build process itself.

View build log

TravisBuddy Request Identifier: 333f3e20-d22f-11ea-9c70-53a116b43db5