create-database: Skip stop-server and flush-memcached on standalone Pg.

If running on a stand-alone PostgreSQL server, then supervisor does
exist -- but `stop-server` is useless, and in fact cannot run because
the Zulip directory may not be readable by the `zulip` user.

Detect if this is an application front-end server by looking for
`/home/zulip/deployments`, and use the stop-server and flush-memcached
from there if it exists.  The `create-db.sql` and
`terminate-psql-sessions` files are still read from the local
directory, but those already have precautions from being from a
non-world-readable directory, and are more obviously important to keep
in sync with the `create-database` script.

scripts/setup/create-database

@@ -29,8 +29,8 @@ if [ "$(su "$POSTGRES_USER" -c "cd / && psql -v ON_ERROR_STOP=1 -Atc \"SELECT 1
 fi
 
 # Shut down all services to ensure a quiescent state.
-if [ -e "/var/run/supervisor.sock" ]; then
-    su zulip -c "$(dirname "$0")/../stop-server"
+if [ -e "/home/zulip/deployments" ]; then
+    su zulip -c "/home/zulip/deployments/current/scripts/stop-server"
 fi
 
 # Drop any open connections to any old database.
@@ -54,6 +54,8 @@ EOF
 fi
 
 # Clear memcached to avoid contamination from previous database state
-"$(dirname "$0")/flush-memcached"
+if [ -e "/home/zulip/deployments" ]; then
+    /home/zulip/deployments/current/scripts/setup/flush-memcached
+fi
 
 echo "Database created"