fix vm-informant dbname: "neondb" -> "postgres" (#4046)

Changes the vm-informant's postgres connection string's dbname from
"neondb" (which sometimes doesn't exist) to "postgres" (which
_hopefully_ should exist more often?).

Currently there are a handful of VMs in prod that aren't working with
autoscaling because they don't have the "neondb" database.

The vm-informant doesn't require any database in particular; it's just
connecting as `cloud_admin` to be able to adjust the file cache
settings.

Dockerfile.vm-compute-node

@@ -54,7 +54,7 @@ RUN set -e \
 
 RUN set -e \
 	&& echo "::sysinit:cgconfigparser -l /etc/cgconfig.conf -s 1664" >> /etc/inittab \
-	&& CONNSTR="dbname=neondb user=cloud_admin sslmode=disable" \
+	&& CONNSTR="dbname=postgres user=cloud_admin sslmode=disable" \
 	&& ARGS="--auto-restart --cgroup=neon-postgres --pgconnstr=\"$CONNSTR\"" \
 	&& echo "::respawn:su vm-informant -c '/usr/local/bin/vm-informant $ARGS'" >> /etc/inittab