Move db.fixed_ip_* tests from DbApiTestCase to FixedIpTestCase

During bp db-api-tests we will create for each model
ModelNameTestCase that contains tests for all methods that works
with this model..

So we shouldn't have DbApiTestCase at all. This patch will move
from DbApiTestCase methods that tests fixed_ip_* methods to already
existing FixedIpTestCase and a little bit improve it.

blueprint db-api-tests

Change-Id: I914c448d26c2b04a9fc3aa9614b7adf6f35b54d0
Merge "Move db.fixed_ip_* tests from DbApiTestCase to FixedIpTestCase"