Code review comment for ~sespiros/ubuntu-security-tools/+git/ubuntu-security-tools:sespiros/fix-build-sources-list

I was proposing something similar but much more complex at:
https://code.launchpad.net/~rodrigo-zaiden/ubuntu-security-tools/+git/ubuntu-security-tools/+merge/420586

yours look much² better, thanks for that.

the only thing I noticed, please correct me if I'm wrong, is that, with that proposed changes, we are going to generate an entry for esm-apps-security for trusty, that does not exist (trusty only uses esm-infra-security)

thanks!