nixos-rebuild: Handle .version-suffix not being writable

Reported by @vcunat.
Use same configuration for S3 as for EBS backed images.
Update to use NixOps in stead on charon, use copy_image method for copying to other regions.
Use stable Nix by default
nscd: Restart if /etc/hosts changes
Update the default binary cache URL to cache.nixos.org
zabbix-server: Add PID file to systemd config.

This is to avoid (in some cases) constant restarting of the Zabbix server, which
causes odds bugs and crashes in the exit handler (if it's too early during
startup).

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
apache-httpd/zabbix: Allow custom configFile.

If option is left by its default value, behaviour is the same as before, using
the configuration file created by the web interface.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
apache-httpd/zabbix: Set max_input_time.

At least the Zabbix 2.x web installer requires max_input_time to be set to 300
seconds. As it doesn't hurt to set it for the 1.x versions, I'm including it
here.

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