Fix 404 error fetching metadata when using DVR

Fix 404 error fetching metadata when using DVR

The metadata agent was unable to find networks
attached to the DVR router because it was only
filtering ports for 'centralized' routers.

To fix the issue, this patch expands the search
filters to include DVR router interfaces during
the network lookup operation.

The extra filter cause no evident performance
loss while serving the request; a different
approach would require to pass the router type
around to narrow down the search filter, but it
sounds like an overkill.

Closes-bug: #1353271

Change-Id: Iefbefa1ff300adad48ab9fc472d5eb1913fbe488