upgrade-postgresql: Do not remove other supervisor configs.

We previously used `zulip-puppet-apply` with a custom config file,
with an updated PostgreSQL version but more limited set of
`puppet_classes`, to pre-create the basic settings for the new cluster
before running `pg_upgradecluster`.

Unfortunately, the supervisor config uses `purge => true` to remove
all SUPERVISOR configuration files that are not included in the puppet
configuration; this leads to it removing all other supervisor
processes during the upgrade, only to add them back and start them
during the second `zulip-puppet-apply`.

It also leads to `process-fts-updates` not being started after the
upgrade completes; this is the one supervisor config file which was
not removed and re-added, and thus the one that is not re-started due
to having been re-added.  This was not detected in CI because CI added
a `start-server` command which was not in the upgrade documentation.

Set a custom facter fact that prevents the `purge` behaviour of the
supervisor configuration.  We want to preserve that behaviour in
general, and using `zulip-puppet-apply` continues to be the best way
to pre-set-up the PostgreSQL configuration -- but we wish to avoid
that behaviour when we know we are applying a subset of the puppet
classes.

Since supervisor configs are no longer removed and re-added, this
requires an explicit start-server step in the instructions after the
upgrades complete.  This brings the documentation into alignment with
what CI is testing.

scripts/setup/upgrade-postgresql

@@ -34,7 +34,7 @@ fi
     crudini --set "$ZULIP_CONF" machine puppet_classes zulip::profile::base,zulip::postgresql_base
     touch "/usr/share/postgresql/$UPGRADE_TO/pgroonga_setup.sql.applied"
 
-    "$ZULIP_PATH"/scripts/zulip-puppet-apply -f --config "$ZULIP_CONF"
+    FACTER_LEAVE_SUPERVISOR=true "$ZULIP_PATH"/scripts/zulip-puppet-apply -f --config "$ZULIP_CONF"
     rm -rf "$TEMP_CONF_DIR"
 )