VPC: fix private mtu of vpc tier

[weizhouapache]

Description

This PR fixes #10001

Steps to reproduce the issue

• update global setting for public/private mtu from 1500 to 1450
• create vpc, the public/private mtu is 1450/1450
• create vpc tier

expected result: public/private mtu is 1450/1450
actual result: public/private mtu is 1450/1500

Types of changes

• Breaking change (fix or feature that would cause existing functionality to change)
• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Enhancement (improves an existing feature and functionality)
• Cleanup (Code refactoring and cleanup, that may add test cases)
• build/CI
• test (unit or integration test code)

Feature/Enhancement Scale or Bug Severity

Feature/Enhancement Scale

• Major
• Minor

Bug Severity

• BLOCKER
• Critical
• Major
• Minor
• Trivial

Screenshots (if appropriate):

How Has This Been Tested?

How did you try to break this feature and the system with this change?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 15.14%. Comparing base (2075918) to head (32ddb6e).
Report is 2 commits behind head on 4.19.

Additional details and impacted files

@@            Coverage Diff            @@
##               4.19   #10257   +/-   ##
=========================================
  Coverage     15.14%   15.14%           
- Complexity    11278    11286    +8     
=========================================
  Files          5408     5408           
  Lines        473803   473816   +13     
  Branches      57819    57822    +3     
=========================================
+ Hits          71743    71775   +32     
+ Misses       394039   394017   -22     
- Partials       8021     8024    +3     

Flag	Coverage Δ
uitests	4.29% <ø> (-0.01%)	⬇️
unittests	15.87% <100.00%> (+<0.01%)	⬆️

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