check if a source NAT IP address is needed before assigning one

[DaanHoogland]

Description

This PR...

Fixes: #10122

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

❌ Patch coverage is 2.70270% with 72 lines in your changes missing coverage. Please review.
✅ Project coverage is 16.23%. Comparing base (ef1aaa0) to head (7d116a1).
⚠️ Report is 6 commits behind head on 4.20.

Files with missing lines	Patch %	Lines
...n/java/com/cloud/network/IpAddressManagerImpl.java	2.70%	71 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##               4.20   #12408   +/-   ##
=========================================
  Coverage     16.23%   16.23%           
+ Complexity    13381    13380    -1     
=========================================
  Files          5657     5657           
  Lines        498947   498953    +6     
  Branches      60555    60564    +9     
=========================================
+ Hits          81025    81027    +2     
- Misses       408889   408890    +1     
- Partials       9033     9036    +3     

Flag	Coverage Δ
uitests	4.03% <ø> (+0.02%)	⬆️
unittests	17.09% <2.70%> (-0.01%)	⬇️

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