Make linux_net use objects for last fixed ip query

This converts the final uses of direct database calls from the
linux_net module by utilizing the calls just added to the
FixedIP object.

Related to blueprint nova-network-objects

Related-bug: 1290568
Change-Id: I58828e9112e43967fe8daa070575ca122041690a
Merge "Make linux_net use objects for last fixed ip query"