Refactor to use IsCompleted instead of IsCompletedDeployment

[knanao]

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

NONE

[pipecd-bot]

Code coverage for golang is 35.69%. This pull request increases coverage by 0.05%.

File	Function	Base	Head	Diff
pkg/model/deployment.go	DeploymentStatus.IsCompleted	--	100.00%	+100.00%
pkg/model/deployment.go	StageStatus.IsCompleted	--	100.00%	+100.00%
pkg/model/deployment.go	Deployment.StageMap	--	100.00%	+100.00%
pkg/model/deployment.go	IsCompletedDeployment	57.14%	--	-57.14%
pkg/model/deployment.go	IsSuccessfullyCompletedDeployment	0.00%	--	+-0.00%
pkg/model/deployment.go	IsCompletedStage	0.00%	--	+-0.00%
pkg/model/deployment.go	Deployment.StageStatusMap	0.00%	--	+-0.00%

[nghialv]

👍

/lgtm

[khanhtc1202]

Here you go
/approve

[pipecd-bot]

This pull request is APPROVED by khanhtc1202.

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