config: firewall: add generic parser for firewall configs

config: firewall: add generic parser for firewall configs

Since the basic format of cluster, host and guest firewall
configurations is the same, we create a generic parser that can handle
the common config format. The main difference is in the available
options, which can be passed via a generic parameter.

Co-authored-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Signed-off-by: Stefan Hanreich <s.hanreich@proxmox.com>
Reviewed-by: Lukas Wagner <l.wagner@proxmox.com>
Reviewed-by: Max Carrara <m.carrara@proxmox.com>