Optimize db.instance_floating_address_get_all method

Optimize db.instance_floating_address_get_all
Was:
Get FixedIps with corresponding instance_uuid
For each FixedIp get associated FloatIp (N request)
Now:
Get only list of FixedIps ids
Get all corresponding FloatingIp with one query (select in)
Get only addresses from FloatingIps (because we are using only it)

Add temporary test for db.instance_floating_address_get_all to
ensure that it works as expected.

Change-Id: Id3c515de0ca35707bbfe46b991c2d7cf591e431b
Refactored tests for db.key_pair_*() functions.

All tests for db.key_pair_*() functions are in KeyPairTestCase class now.

blueprint test-db-api

Change-Id: I92358bcda157c95290b31e3b7997e3432a65b404
Merge "Optimize db.instance_floating_address_get_all method"
Merge "Refactored tests for db.key_pair_*() functions."