Code review comment for ~sespiros/ubuntu-security-tools/+git/ubuntu-security-tools:fix-release-info

I agree that it seems like a big change but there were 2 ways to fix the changelog/repo commands bug:
1) revert my previous fix; that means that the release field will keep not detecting ESM releases properly, the ESM chroot naming standardization was just a case that worked as a motivation to make all these changes.
2) keep the new field (base_release) and differentiate code that needs a full release name from code that needs just the base release (this patch)

Regarding the name, "distribution" could be an alternative one but I haven't seen that used in any of our tools. cve_lib for example uses the term "parent" https://git.launchpad.net/ubuntu-cve-tracker/tree/scripts/cve_lib.py#n113 to indicate the same thing, so that could be a candidate.

If we wanted to be pedantic, according to
- https://docs.ubuntu.com/landscape/en/repositories and
- https://wiki.ubuntu.com/DevelopmentCodeNames

technically a release is "Ubuntu" plus the version "X.YY" (and I guess the minor versions as well). "series" could also be the term we could use instead of "base_release" ("parent" in cve_lib) and maybe "subproject" instead of "release" (subproject is the term cve_lib uses).