use reflect pkg to break the limit of for-range by dynamic select/case

[howieyuen]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #239

Special notes for your reviewer:

Does this PR introduce a user-facing change?

refactor: use reflect pkg to break the limit of for-range by dynamic select/case set

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

None

[coveralls]

Pull Request Test Coverage Report for Build 4132792741

• 44 of 45 (97.78%) changed or added relevant lines in 1 file are covered.
• 7 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.1%) to 71.886%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/engine/operation/watch.go	44	45	97.78%

Files with Coverage Reduction	New Missed Lines	%
pkg/engine/operation/parser/parser.go	3	75.0%
pkg/engine/operation/parser/spec_parser.go	4	66.04%

Totals
Change from base Build 4132501405:	0.1%
Covered Lines:	4646
Relevant Lines:	6463

---

💛 - Coveralls

[markliby]

lgtm