Convert network/api.py fixedip calls to use FixedIP object

This makes the network/api.py module use the FixedIP object for
fetching fixedip information instead of direct database access.
There was not complete test coverage for these calls, so it is added
here.

Note that this includes a change to the FixedIP object, which allows
for the case that fixed_ip_get_by_floating_address() can return
None. This is technically an RPC-visible change that would normally
require a version bump. However, I don't think it's worth bumping
the version here since this bug would cause calls to explode in a
non-useful way, thus preventing callers from being able to tolerate
the case where the DB call returns None.

Change-Id: Id8a9c8d2d10ddfaf0990258ac496da7c48a9b28c
Merge "Convert network/api.py fixedip calls to use FixedIP object"