Swap the relationship between Vrfs and Networks

Before this commit, the Network model had a one-to-many relationship
with the Vrf model. This commit swaps the direction of the relationship.
Add schema migrations
Fix migrations after Vrf-Network relationship flip
Fix some recently broken functions
Fix a test and another broken function
Range.get_allowed_clients() should return a list
Iterate through iterable things
Merge pull request #277 from drkitty/flip_vrf-network_relationship

Flip Vrf–Network relationship. Fully Functional, all tests pass.