Metadata support for NVP plugin

Bug #1121119

Allows the NVP plugin to leverage the metadata proxy, by creating an
ad-hoc topology for allowing access to a metadata proxy from a NVP
router leveraging existing agents.

This patch also removes previous code for metadata support in the
NVP plugin, which was based on DHCP Option 121. This is now provided
by the dhcp agent as well.

Change-Id: If37ef388e063f40bb06908ee2f72c431f29dac31
Merge "Metadata support for NVP plugin"