Clean up blackhole routes on the provider router

* When deleting a subnet, we need to clean up the blackhole
  routes that are on the subnet.
* Added a route filter to return the routes to be deleted.

Signed-off-by: Joe Mills <joe@midokura.com>

Fixes MN-1119
Merge pull request #77 from JoeMido/MN-1119v2

Clean up blackhole routes on the provider router