Fix race in get_network(s) in OVS plugin

Load network bindings eagerly with networks.
Otherwise a different db query could try to fetch network bindings
for already deleted networks. The issue is reproducible with
concurrent tempest network API tests.

Closes-Bug: 1263686
Change-Id: I0521ab162220c66a62491ff8e7e4a9f6d7bef6c4
Merge "Fix race in get_network(s) in OVS plugin"