Adding port variable to server set up

Signed-off-by: Alexis Pentori <alexis@status.im>

templates/webhook.service.j2

@@ -9,6 +9,7 @@ After=docker.service
 User={{ github_webhook_service_user }}
 ExecStart={{ github_webhook_script_dir }}/server.py \
     --repo-url={{ github_webhook_repo_url | mandatory }} \
+    --port={{ github_webhook_port }} \
     --repo-branch={{ github_webhook_repo_branch | mandatory }} \
     {{ github_webhook_dest_path | mandatory }}
 WorkingDirectory={{ github_webhook_script_dir }}