Cache neutron subnets before deleting

 * When deleting a network, neutron will also delete all subnets
   associated with that network. This causes a problem because
   we rely on information about the subnets in neutron for
   cleaning up. The solution is to just cache that information
   before.

Signed-off-by: Joe Mills <joe@midokura.com>

Fixes MN-2328
Fixes MN-2324