Add metadata_network, which make metadata proxy work with multiple networks when isolated_metadata is enabled and namespace is disabled 

This patch added metadata_network flag, which is used to specify network used to get metadata when
enable_isolated_metadata=True and use_namespaces=False. With this flag, the other networks(other than metadata network) will not serve metadata proxy, which means VM will not get the metadata through the other networks. If a guest image does not contain special network settings, the network specified in this flag better be the first network(assigned to eth0 of VM) because most of the guest images enable eth0 only by default.