refactor: preview&apply cmd implementation follow the command cmd pattern

[adohe]

What type of PR is this?

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #

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Does this PR introduce a user-facing change?

Additional documentation e.g., design docs, usage docs, etc.:

[coveralls]

Pull Request Test Coverage Report for Build 8679405118

Details

• 212 of 406 (52.22%) changed or added relevant lines in 4 files are covered.
• 49 unchanged lines in 5 files lost coverage.
• Overall coverage decreased (-1.5%) to 55.512%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/cmd/cmd.go	0	2	0.0%
pkg/cmd/meta/meta.go	0	10	0.0%
pkg/cmd/preview/preview.go	62	132	46.97%
pkg/cmd/apply/apply.go	150	262	57.25%

Files with Coverage Reduction	New Missed Lines	%
pkg/engine/operation/watch.go	1	85.93%
pkg/cmd/generate/generator/generator.go	9	0.0%
pkg/cmd/apply/apply.go	9	55.35%
pkg/cmd/preview/preview.go	9	43.97%
pkg/cmd/generate/generate.go	21	0.0%

Totals
Change from base Build 8679391272:	-1.5%
Covered Lines:	6828
Relevant Lines:	12300

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💛 - Coveralls

[liu-hm19]

lgtm