Clean up db network db calls for fixed and float

Use a join instead of a subquery for fixed_ip_get_by_floating_address
and floating_ip_get_by_fixed_address. Adds test to verify behavior
is the same.

Change-Id: Iab00142d477308c354777a2c64b6d03a21184585
Merge "Clean up db network db calls for fixed and float"