Update git submodules

* Update docs/submodules/so.git from branch 'master'
  to e28e37139a19f942ef683e891389c055fffdc224
  - Add test for deleting slice service
    
    Issue-ID: SO-2368
    
    Signed-off-by: zm330 <zhangminyj@chinamobile.com>
    Change-Id: Icd892109aeea94d544c4d0efdc108ffb4d7991cb
    
1 file changed