### What problem does this PR solve?
Fix minor code quality issues:
1. Fix typo in assertion error message: "Can't fine" → "Can't find"
2. Remove duplicate line in common/connection_utils.py
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
- [x] Refactoring
### What problem does this PR solve?
1. Update RetCode to common.constants
2. Decouple the admin and API modules
### Type of change
- [x] Refactoring
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Signed-off-by: Jin Hai <haijin.chn@gmail.com>