Add FixedIP Object implementation
Add FixedIP Object implementation

This adds the FixedIP object implementation to allow nova-network
to perform these operations with an object.

Related to nova-network-objects

Change-Id: I262b316e3c509f3a5ea6629e74b62011c7d35ba4

citrix-openstack-build/oslo.versionedobjects,openstack/oslo.versionedobjects