visual studio team services build - How to specify different connection string for a Sql Server Test Project

I have a SQL Server unit testing project, the connection string for the database is located in the app.config file as follows

    <configuration>
    <configSections>
        <section name="SqlUnitTesting" type="Microsoft.Data.Tools.Schema.Sql.UnitTesting.Configuration.SqlUnitTestingSection, Microsoft.Data.Tools.Schema.Sql.UnitTesting, Version=14.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />
    </configSections>
    <SqlUnitTesting>
        <!--<DatabaseDeployment   DatabaseProjectFileName="..\..\..\AklAlbait.Database\AklAlbait.Database.sqlproj"
            Configuration="Debug" />-->
        <DataGeneration ClearDatabase="true" />
        <ExecutionContext Provider="System.Data.SqlClient" ConnectionString="Data Source=.;Initial Catalog=AklAlbait;Persist Security Info=True;User ID=sa;Pooling=False;Connect Timeout=30"
            CommandTimeout="30" />
        <PrivilegedContext Provider="System.Data.SqlClient" ConnectionString="Data Source=.;Initial Catalog=AklAlbait;Persist Security Info=True;User ID=sa;Pooling=False;Connect Timeout=30"
            CommandTimeout="30" />
    </SqlUnitTesting>
</configuration>

Now when i try to create a build definition to run on VS team services (VSO), i need away to specify a different connection string to be used, how can i do this?? in the case of Team Foundation Server we can add another config file named "BuildComputer.app.config", where BuildComputer is the name of the computer on which your build agent runs. (http://ift.tt/1Lrh6BB). I need something that works on the online team foundation build.