Report warning when config sections are not used
It's possible a variant is excluded via tree_variants option and the
section does not match anything. It can be confusing to users why
nothing is happening. This patch lets Pungi log all unmatched patterns.
Fixes: https://pagure.io/pungi/issue/692
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>