rename lsbRelease to prettyName and set it from os-release as well

Summary:
previously when lsb_release was not present or working we'd fall back
to os-release, but then not track the pretty name leading to the report
containing no Distribution value.
the more aptly named info variable is now set by both lsb_release and
os-release logic, ensuring that in both scenarios we have the distro
recorded in the report.

systeminformationtest now also checks that the pretty name is set when
testing on os-release (without lsb_release)

this also will allow us to eventually move to prefer os-release instead of
calling the external lsb_release script.

Test Plan: builds & passes

Reviewers: #plasma, apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22141