Allow 'copr' user to run 'sign' command

See https://pagure.io/copr/copr/issue/636

By default only root can run the `sign` command. This
check is applied within obs-signd code. We need to
allow regular user in the config, see `man sign.conf`.

Also /usr/bin/sign is owned by root:obsrun with
-rwsr-x--- hence we need to add a user to the obsrun group.

roles/copr/backend/tasks/main.yml

@@ -7,7 +7,7 @@
 
 # pre-create copr user and group with predefined uid and gid
 - group: name=copr gid=986
-- user: name=copr group=copr uid=989
+- user: name=copr group=copr uid=989 groups=obsrun
 
 - name: install copr-backend and copr-selinux
   dnf:

roles/copr/backend/templates/sign.conf

@@ -1 +1,2 @@
 server: {{ keygen_host }}
+allowuser: copr