Make db_manager.DaoSession private

this is a first step toward a Session object for all dao functions

a mocked_dao_session decorator has been added to the db_manager module to
decorate test functions that need a mocked session instance.

The interesting stuff in this commit is in xivo_dao.helpers.db_manager and in xivo_dao.tests.test_dao
other changes are mostly replacing DaoSession patching with @mocked_dao_session