Description of problem:
   Since non-scalable app do not support to add mysql and postgresql at the same time.After user add mysql to an app, the postgresql cartridge will be moved from the app's cartridge select list to the cartridge conflict list. 

   However, for scalable app, we support to add mysql and postgresql to the app at the same time now. So we should not remove the postgresql cartridge from the cartridge select list for scalable app after user add mysql to the app.

Version-Release number of selected component (if applicable):
devenv_2431

How reproducible:
always

Steps to Reproduce:
1.Go to website, and create a scalable app.
2.Add mysql to the app.
3.Check the cartridge select list of the app.
4.Try to add postgresql in the page: /app/console/applications/spring/cartridge_types/postgresql-8.4
  
Actual results:
At step 3, the cartridge postgresql would be removed from the cartridge select list , and stayed in the conflict list. 
At step 4, the postgresql can be added successfully.

Expected results:
At step 3, the cartridge postgresql should still in the cartridge select list.

Additional info: