network: Implement remove_bridge(), other clean-ups.
rootwrap: Add sysctl to the list
network: Implement initialize_gateway_device(), helper routines

This commit brings a bunch of helper routines for ifconfig/netstat
manipulation, which also simplify ensure_bridge() and friends..
network: Stub for update_dhcp

With this change we're able to run the whole network init sequence (bridge and
gateway init on the host FreeBSD) and keep nova-network process alive and
communicating with the cluster control node.