Fix migration 210 tests for PostgreSQL

We check index in table in _check_210() and _post_downgrade_210()
methods in a such way:

self.assertIn(('project_user_quotas_user_id_deleted_idx',
               ['user_id', 'deleted']), index_data)

But we have no possibility to get the order of columns due to SQLA
issue in reflection of indexes for PostgreSQL.
See http://www.sqlalchemy.org/trac/ticket/2767
So columns order in `index_data` variable can be wrong and test
fails in such case.

To fix this we should store colums as sets (instead of lists) if
we run tests on PostgreSQL.

Fixes bug 1213921

Change-Id: I9e10b325940becfdcff6b13fcaa4146731126a60
Merge "Fix migration 210 tests for PostgreSQL"