Fix pod assignment issue when pod already has a node assigned

[chaunceyjiang]

What type of PR is this? /kind bug

What this PR does / why we need it: Fix pod assignment issue when pod already has a node assigned Which issue(s) this PR fixes: Fixes #488

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Flag	Coverage Δ
unittests	27.09% <100.00%> (+0.18%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
pkg/scheduler/webhook.go	56.00% <100.00%> (+9.19%)	:arrow_up:

[chaunceyjiang]

/cc @wawa0210 @archlitchi @lengrongfu PTAL.

[wawa0210]

Codecov Report

Can you solve the Codecov Report UT coverage issue?

[chaunceyjiang]

Can you solve the Codecov Report UT coverage issue?

Done.

[lengrongfu]

/LGTM

[wawa0210]

/lgtm thanks your contribution