rhsm_repository: refactor parsing of "subscription-manager repos" output (#6837)
Simplify a bit (and possibly speed it up a little) the parsing of the
output of `subscription-manager repos --list`:
- simplify skipping the lines that are not interesting: check the first
character only, as it is enough to determine whether it contains
repository data or not
- check the start of each line manually, rather than with regexp: a
simple slice + lstrip() gives the same result
(cherry picked from commit e0324cdc90)
Co-authored-by: Pino Toscano <ptoscano@redhat.com>
