puppet nftables

Allow creating a totally empty firewall

By setting `nftables::inet_filter` and `nftables::nat` to `false`
users can now start off from a totally empty firewall and add the
tables, chains and rules they'd like.

The default skeleton for inet-filter, ip-nat and ip6-nat is kept...

New parameter noflush_tables to selectivly skip flush

Introduces a new structured fact nftables

```yaml
nftables:
tables:
- inet-filter
- ip-nat
- ip6-nat
- inet-f2b-table
```

By default the nft script will continue to contain `nft flush ruleset`...