Make nova-network use Network object for deleting networks

This makes nova-network manager use the Network object when
deleting networks. Since it results in the same underlying call
to the database, it doesn't affect tests at all.

Related to blueprint nova-network-objects

Change-Id: If45020b8264f05b12657bb27f836b9fc60964cd1
Refactor return value of fixed_ip_associate calls

This makes the fixed_ip_associate* calls more object-friendly.
Rather few things outside of the tests care about the return value of
these methods, so this is less invasive than it looks. All of the
tests and manager methods that this touches will be replaced
as part of the objectification going forward, so this is only
transitional.

Related to blueprint nova-network-objects

Change-Id: I4764fa0bfca355377d2642ddf3fbfe17cf5b761e
Make fixed_ip_get_by_address() take columns_to_join

This will let the FixedIP object make some significantly more
efficient queries by asking for network and/or instance at the
same times as the FixedIP itself.

Related to nova-network-objects

Change-Id: I7ae971469585f072cd039f30faf71a4d48c9fd04
Merge "Make nova-network use Network object for deleting networks"
Merge "Refactor return value of fixed_ip_associate calls"
Merge "Make fixed_ip_get_by_address() take columns_to_join"