Add upstream_tag_template config to packit

Since we use v0.x.y form for release tags, Packit requires a config
named upstream_tag_template to be able to extract version number from
the tag.

Signed-off-by: Mark Kemel <mkemel@redhat.com>

.packit.yml

@@ -4,6 +4,7 @@ issue_repository: https://github.com/eclipse-bluechi/bluechi
 specfile_path: bluechi.spec
 upstream_package_name: bluechi
 downstream_package_name: bluechi
+upstream_tag_template: v{version}
 
 update_release: false