ansible-scylla-monitoring: make start wrapper idempotent for systemd unit

The 'Enable scylla-monitoring at boot' task sets state: started on the
scylla-monitoring.service unit right after the stack was already
started manually via the same start wrapper a few tasks earlier. Since
the unit was freshly installed (and thus inactive from systemd's point
of view), enabling it with state: started re-runs ExecStart
(start-monitoring.sh -> start-all.sh) against containers that are
already up, so start-all.sh fails with a container/name conflict and
systemd reports 'control process exited with error code'.

Make start-monitoring.sh.j2 tear down any already-running stack (via
the already-idempotent stop wrapper) before starting, so it is safe to
invoke back-to-back from the manual start task and then again from the
systemd unit.

Co-authored-by: Manjot Singh <ManjotS@users.noreply.github.com>

Author: cursoragent

ansible-scylla-monitoring/templates/start-monitoring.sh.j2

@@ -6,4 +6,9 @@ if [ -f "$ENV_FILE" ]; then
   # shellcheck disable=SC1090
   . "$ENV_FILE"
 fi
+# Tear down any already-running stack first so this script is safe to run
+# more than once in a row (e.g. once from the deploy task, then again when
+# the systemd unit is started right after) instead of failing on container
+# name conflicts left over from the previous run.
+{{ scylla_monitoring_stop_script_path | quote }} || true
 {{ start_command }}