kvm: Implement whitelist firewall

Sources of domains and IPs are:

 * default hardcoded in template/whitelist-domains-default
 * /etc/resolv.conf
 * provided in the request
 * provided in the special downloadble repository

Then they are parsed with dnsresolver and .slapos-whitelist-firewall file is
produced with list of IPs to be whitelisted.

This allows slapos.core whitelistfirewall manager to lock-down the partition
to only whitelisted list of IPs.