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
Use comments rather than no-op string statements

This change replaces a few no-op string statements with regular
comments.  While there was no harm in the previous use of strings for
comments, this allows us to re-enable the corresponding pylint check
which may catch genuinely unintended cases.

Change-Id: I796a059292e26c4df75c54f095d9e20e99187c98
Merge "Use comments rather than no-op string statements"
Merge "Refactor test_migration"