Query packages from repo by repo label

Each repo can be labeled with a specific label, think tags. This
way multiple repositories can be grouped together. This patch
adds a query parameter to search outdated packages on all repositories
with the same label.

The labeling can be used, e.g. to group mirrored repositories together
so you can scan your whole environment for outdated packages from all
repository sources.

Signed-off-by: Christoph Kassen <c.kassen@telekom.de>