Add time_type dictionary to test_migrations

A number of postgresql migration tests fail because the test case
is verifying that an sqlalchemy.types.DATETIME column was created.
When the column type is checked, however, postgresql reports the type as
sqlalchemy.types.TIMESTAMP.  The mismatch causes numerous
test cases to fail for postgresql.

This creates a dictionary for each engine being tested to handle the
different time types.

Change-Id: Ic9921ff9fda001a66c0ea3a6a75c8adefa60b107
Closes-bug: 1262426
Merge "Add time_type dictionary to test_migrations"