sdn: zones: extract function that reads datacenter config

sdn: zones: extract function that reads datacenter config

The datacenter_config() functions in SDN::Zones::Plugin is a simple
wrapper that reads datacenter.cfg via cfs.
This allows mocking datacenter.cfg in tests.

Signed-off-by: Stefan Lendl <s.lendl@proxmox.com>
Reviewed-by: Max Carrara <m.carrara@proxmox.com>
Tested-by: Max Carrara <m.carrara@proxmox.com>
Reviewed-by: Stefan Hanreich <s.hanreich@proxmox.com>
Tested-by: Stefan Hanreich <s.hanreich@proxmox.com>