Refactor _make_subnet_dict to avoid issuing unnecessary queries

Use joined loads for attributes dns_nameservers and host_routes.

As a result, particular scenarios like restarting DHCP agent
could benefit from improved server-side performance.

Change-Id: I6470356b601e2fcf74c7e0a6df438cef7099e9fe
Closes-Bug: #1374044
Merge "Refactor _make_subnet_dict to avoid issuing unnecessary queries"