Adding Portal Configuration Only Mode configuration

[CrowleyRajapakse]

Issue

Fixes wso2/api-manager#2687

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 15 lines in your changes are missing coverage. Please review.

Project coverage is 47.30%. Comparing base (45a43e6) to head (49aba2f).
Report is 52 commits behind head on master.

Files	Patch %	Lines
.../apimgt/rest/api/publisher/v1/dto/SettingsDTO.java	0.00%	9 Missing ⚠️
...ava/org/wso2/carbon/apimgt/impl/utils/APIUtil.java	0.00%	5 Missing ⚠️
...lisher/v1/common/mappings/SettingsMappingUtil.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #12342      +/-   ##
============================================
- Coverage     47.43%   47.30%   -0.13%     
+ Complexity     4161     4133      -28     
============================================
  Files          1840     1840              
  Lines        136499   136513      +14     
  Branches      19759    19761       +2     
============================================
- Hits          64742    64584     -158     
- Misses        64126    64311     +185     
+ Partials       7631     7618      -13     

Flag	Coverage Δ
integration_tests	42.12% <0.00%> (-0.08%)	⬇️
unit_tests	20.16% <0.00%> (+0.01%)	⬆️

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