Refactor test_migration

test_migration contains some methods like overriding compare_server_default
and compare_foreign_keys that was added directly in ModelsMigrationsSync
class in oslo.db only in 1.1.0.

Also as now migrations refactoring is fully finished, there is no
need to have separate classes for each plugin.

Closes-bug: #1393691

Change-Id: I65b40fe52c2c6f4bae88e125bd1ee501aaa24114
Merge "Refactor test_migration"