Block database access in nova-network binary

This mirrors the code we have in cmd/compute.py to wedge the database
module so that we can't add any more direct calls, and will highlight
any stragglers.

Related to blueprint nova-network-objects

Change-Id: Ib12415d84de950265626b4a804f33717203d9765
Related-bug: 1290568
Merge "Block database access in nova-network binary"