Previously, an internal metadata network is created
for a router whenever a subnet is attached to this router.
The purpose of this internal network is to help processing
metadata requests from instances on DHCP-disabled networks.
This commit adds a config option to create internal metadata
networks only when a DHCP-disabled subnet is attached to a router.
This will help saving system resources because each metadata
network consumes one DHCP name space.
Change-Id: Ia56050b3f431dbd65bb39da29ba6dbf8e62e36ea
61c19f3e9c