Add missing tests for db.floating_ip_* methods

There was no tests in test_db_api for db.floating_ip_*
methods.

Add tests to ensure that all works.

Remove tests for db.flating_ip_* from DbApiTestCase.

Change return value of db.floating_ip_create()
This method is used only for tests. To simplify testing we
should return floating_ip_ref (all columns) instead of address.

blueprint db-api-tests

Change-Id: I37ee93c1d054ad736aa02a43be867f809448b172
Merge "Add missing tests for db.floating_ip_* methods"