suite: Filter YAML fragments for distro, version

suite: Filter YAML fragments for distro, version

The behavior of --distro{,-version} was never well-defined. With this change,
they can independently be used to provide a default for jobs which don't
otherwise specify a value; and more importantly, perform a post-merge filter
looking at each job's values, rejecting those which don't match. This behavior
should be more intuitive and more easy to understand.

Signed-off-by: Zack Cerza <zack@cerza.org>