Fix non-deterministic error in unit test

next_auto_assign_ip is set in the cidr's constructor so depending on
the subnet dictionary key memory layout next_auto_assign_ip would be
set to 0 as opposed to 2 if the cidr's constructor was invoked after
next_auto_assign_ip was manually set. This was fixed by updating
next_auto_assign_ip after the subnet model was created.
Merge pull request #66 from asadoughi/master

Fix non-deterministic error in unit test
Extend subnets to allow for nested IPPolicy creation

- ensures IPPolicy fits into Subnet CIDR
- ensures IPPolicy has valid CIDR iteself
- ensures IPPolicy CIDR and inverse of allocation_pools are merged
  to create one IPPolicy, since Subnet can only have one associated
  to it