Add metadta_network flag

This option is used to specify network used to get metadta when
enable_isolated_metadata=True and use_namespaces=False. The network
specified in this flag must be the same with 'pre_defined_network'
in Nova, because this network should be assigned to eth0 inside the VM.
Merge pull request #17 from hyunsun/UCLOUDNG-761

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