isolated network metadata does not work with nvp plugin

isolated network metadata does not work with nvp plugin

This patch removes the not self.conf.enable_metadata_network
check before adding METADATA_DEFAULT_IP to ip_cidrs. This result
of this is that the dhcp agent interface will have
169.254.169.254/16 on it even if the network is
enable_metadata_network=True which doesn't matter and happens anyway if one
sets enable_meta_network=False. This change only effects the nvp plugin
as this is the only one that uses the enable_metadata_network flag.

Change-Id: Ifb3b28daed9e3bfc61bb0d8a48c40d027d0e2a51