Use separate database when running on sqlite3

To avoid ugly SQLite3::BusyException: database is locked when sharing
the database with ActiveRecord

To have cleaner look on what's it about, see http://ci.theforeman.org/job/test_katello_core/1030/database=sqlite3,ruby=1.9.3/testReport/(root)/Katello__ActivationKey__in invalid state/test_0004_should_be_invalid_if_environment_in_another_org_is_specified/