Move telegraf_output from global pillar to telegraf pillar

The Telegraf backend selector lived at global.telegraf_output but it is
a Telegraf-scoped setting, not a cross-cutting grid global. Move both
the value and the UI annotation under the telegraf pillar so it shows
up alongside the other Telegraf tuning knobs in the Configuration UI.

- salt/telegraf/defaults.yaml:    add telegraf.output: BOTH
- salt/telegraf/soc_telegraf.yaml: add telegraf.output annotation
- salt/global/defaults.yaml:      remove global.telegraf_output
- salt/global/soc_global.yaml:    remove global.telegraf_output annotation
- salt/vars/globals.map.jinja:    drop telegraf_output from GLOBALS
- salt/firewall/map.jinja:        read via pillar.get('telegraf:output')
- salt/postgres/telegraf_users.sls: read via pillar.get('telegraf:output')
- salt/telegraf/etc/telegraf.conf: read via TELEGRAFMERGED.output
- salt/postgres/tools/sbin/so-stats-show: update user-facing docs

No behavioral change — default stays BOTH.

salt/postgres/tools/sbin/so-stats-show

@@ -24,7 +24,7 @@ Shows the most recent CPU, memory, disk, and load metrics for each host
 from the so_telegraf Postgres database. Without an argument, reports on
 every host that has data. With a host, limits output to that one.
 
-Requires: sudo, so-postgres running, global.telegraf_output set to
+Requires: sudo, so-postgres running, telegraf.output set to
 POSTGRES or BOTH.
 EOF
   exit 1
@@ -56,7 +56,7 @@ so_psql() {
 }
 
 if ! docker exec so-postgres psql -U postgres -lqt 2>/dev/null | cut -d\| -f1 | grep -qw so_telegraf; then
-  echo "Database so_telegraf not found. Is global.telegraf_output set to POSTGRES or BOTH?"
+  echo "Database so_telegraf not found. Is telegraf.output set to POSTGRES or BOTH?"
   exit 2
 fi