check namespace permission and retrieve namespace id from db

[nobodyiam]

What's the purpose of this PR

check namespace permission and retrieve namespace id from db

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[codecov-io]

Codecov Report

Merging #2973 into master will decrease coverage by 0.01%.
The diff coverage is 50.00%.

@@             Coverage Diff              @@
##             master    #2973      +/-   ##
============================================
- Coverage     51.20%   51.19%   -0.02%     
+ Complexity     2229     2228       -1     
============================================
  Files           432      432              
  Lines         13378    13381       +3     
  Branches       1373     1374       +1     
============================================
  Hits           6850     6850              
- Misses         6051     6053       +2     
- Partials        477      478       +1     

Impacted Files	Coverage Δ	Complexity Δ
...llo/portal/controller/ConfigsExportController.java	8.16% <ø> (ø)	1.00 <0.00> (ø)
...p/framework/apollo/portal/service/ItemService.java	47.87% <50.00%> (-0.27%)	16.00 <0.00> (ø)
...apollo/portal/entity/model/NamespaceTextModel.java	82.60% <0.00%> (-4.35%)	13.00% <0.00%> (-1.00%)

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[coveralls]

Coverage decreased (-0.005%) to 54.764% when pulling ff07e39 on nobodyiam:misc-change into 9f4bac4 on ctripcorp:master.