Interfaces API Clean up

As we've evolved how Nova interfaces and Melange interact. We've iterated over the API and now probably need to do some clean up before finalization. These change would apply to all REST verbs on the calls (not just POST but GET, DELETE, etc.) Here is the proposal:

Current API:

PUT /ipam/instances/{instance_id}/interfaces/
Expects a collection of interfaces in the request body. Optionally allocates ips if network details are present for each interface.
Repeated invocation will delete existing interfaces and add new ones.
Usecase is for allocating all interfaces of an instance when spinning up a new instance.

Proposed API:

POST /ipam/instances/{instance_id}/interfaces/
Expects a single interface in the request body. Optionally allocates ips.
Repeated invocation creates new interfaces.
Usecase is for existing instances to create new interfaces.

POST /ipam/instances/{instance_id}/interfaces/ip_addresses
Would allocate and additional ip from the block associated with the network this interface is connected to
Repeated invocations would add more IPs

Old APIs:

1) Interface API:
POST /ipam/interfaces.
Behavior is same as the proposed /ipam/instances/{insance_id}/interfaces, except that instance_id is taken through request body and is optional.

2) IpAllocations API:
POST /ipam/tenants/{network_owner_tenant}/networks/{network_id}/interface/{interface_id}/ip_allocations
Allocates ips for existing interfaces. If interface is not found, will create one on the fly.

API 1) is redundant once the proposed API is in place.
API 2) would still be necessary if we create interfaces without allocating ips and at a later stage allocate ips.

But I don't see the usecase for creating interfaces in melange and allocating ips later. Unless there is a reasonable usecase we can remove API 2)