Description of problem:
Mysql and postgresql cartridge can be embedded to a same app at the same time

Version-Release number of selected component (if applicable):
On fork_ami_US2105_133

How reproducible:
Always

Steps to Reproduce:
1. Create an application
2. embed mysql to app, and then embed postgresql to app

  
Actual results:
Mysql and postgresql are added to the same app.
Application Info
================
phpapp
    Framework: php-5.3
     Creation: 2012-09-24T07:18:22-04:00
         UUID: 5c215e0ff1364949b2a6886833013710
      Git URL: ssh://5c215e0ff1364949b2a6886833013710.rhcloud.com/~/git/phpapp.git/
   Public URL: http://phpapp-fk2105t1.dev.rhcloud.com/

 Embedded: 
      postgresql-8.4 - Connection URL: postgresql://127.0.255.129:5432/
      mysql-5.1 - Connection URL: mysql://127.0.255.129:3306/


Expected results:
Should allow only one database embedded.

Additional info:
Our devenv now allow only one database, mysql and postgresql can't be embedded at the same time to one app.