Session cleanup for db.floating_ip_* methods

Session objects shouldn't be used in public db.api methods
because db.api should be independent of db implementation.

Remove session parameter from:
  floating_ip_create
  floating_ip_get_by_fixed_ip_id

I didn't remove session parameter from floating_ip_count_by_project
because it causes changes in nova.quota. So it will be done in
separated patch.

Cleanup test for floating_ip_count_by_project removed 3rd
parameter (it should be session, but it is not required).

blueprint db-session-cleanup

Change-Id: Ifdf9edb17746a8e5b486317c917ea69016a2b99d
Merge "Session cleanup for db.floating_ip_* methods"