* Dont make 2 * instances queries when generating dhcp leases
* Uses some nasty joins to minimize necessary changes
* Adds test to verify functionality
* Uses old sqlalchemy compatible join syntax
* Uses vif_id as nw identifier for single_host_gateway mode
* Cleans up tests in linux_net
* Fixes bug 959378
Change-Id: I1c74d114dd7caad1781180585bb65691e536bcf9