Apt source list in testing(stable) image should match Debian default list

Debian testing has the following lines by default in apt sources:

• deb mirror stretch main
• deb-src mirror stretch main
• deb http://security.debian.org/ stretch/updates main
• deb-src http://security.debian.org/ stretch/updates main

And Debian stable has the following:

• deb mirror jessie main
• deb-src mirror jessie main
• deb http://security.debian.org/ jessie/updates main
• deb-src http://security.debian.org/ jessie/updates main
• deb mirror jessie-updates main
• deb-src mirror jessie-updates main