Fix straggling uses of direct-to-database queries in nova-network

There were a few remaining calls directly to the database module
left in nova-network. These would fail because of the wedge we
put in place specifically to catch this case. However, we don't
test VlanManager in the gate, which means we didn't catch them
until now.

This required adding two more methods, one each to Network and
NetworkList, but they are both extremely simple.

Change-Id: Iecc382074f060da1bd6f740d7bde0b20a0be2fcf
Closes-bug: #1304724
(cherry picked from commit 275a165cf0e74112d7ec9addacb1f84d703977c8)