znc: fix openFirewall option

The current version is broken:
- there's no `openFirewall` attribute directly in the `cfg` set
- the `port` option is an attribute of the `confOptions` set

I used the proper attribute for the firewall port and moved the `openFirewall`
option directly up to the `services.znc` set, as it's rather a general option
for the whole service than a znc-specific option (which are located inside the
`confOptions` set).