Fix postgres host

[Pezhvak]

Using helm chart, the default config does not work, after 3-hours of investigation, i found out that the issue is here:

docker-zulip/entrypoint.sh

Line 454 in 08c58e1

DB_HOST=${1:-$(su zulip -c "/home/zulip/deployments/current/scripts/get-django-setting REMOTE_POSTGRES_HOST")}

This line returns 127.0.0.1 and in kubernetes pg_isready is not gonna work, it should see the service name which is zulip-postgresql.

I've added that into values.yaml since it's required. if you think there is a better way to do this let me know.