zabbix-server: Make it easier to use peer auth.

Quoting from the manual about DBHost:

```
In case of MySQL localhost or empty string results in using a socket. In case of
PostgreSQL only empty string results in attempt to use socket.
```
https://www.zabbix.com/documentation/2.0/manual/appendix/config/zabbix_server

With this commit we should avoid some race conditions in systemd, because if the
host is set to "", there is no condition that postgresql has to be started prior
to the Zabbix server.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
zabbix-server: Swap order of database population.

Starting with Zabbix 2.0 the order of data imports is important[*] and will lead
to errors if not done in the right order. Zabbix 1.8 works fine with the swapped
order as well, so this change shouldn't affect any pre-2.0 users.

[*] https://www.zabbix.com/documentation/2.0/manual/appendix/install/db_scripts

Signed-off-by: aszlig <aszlig@redmoonstudios.org>