Fix InstanceActionTestCase on PostgreSQL/MySQL

test_instance_action_get_by_instance_and_action() of
InstanceActionTestCase mistakenly uses the same value
of request_id and then relies on SQLite specific order
of retrieved rows to grab a required value. This causes
test to fail on MySQL and PostgreSQL.

Blueprint: db-api-tests-on-all-backends

Co-Authored-By: Roman Podoliaka <rpodolyaka@mirantis.com>
Change-Id: I6e3e100627ef7efc31c41aaa5a38d45e828fcf39
Merge "Fix InstanceActionTestCase on PostgreSQL/MySQL"