Drop hostcfg mutation when configuring bonding

Mutating the passed host configuration was introduced when adding support for bonding, see git-commit 334d2683. This mutation is moot because the set "network_interface" field is anyway ignored when doing bonding.

The follow-up change to drop the "network_interface" field transformed this mutation into an append on the "network_interfaces" field, see git-commit c3ccf555. There is still no obvious reason for keeping this mutation. The relevant part of the bonding configuration is to read target interface-names and use netlink. In other words, "network_interfaces" is not read again after bonding.

Part of stprov#60 (closed).