Cleaning state pollution in si_app

[sturmianseq]

This PR aims to improve test reliability of test test_plugin_hdfs_add by cleaning state pollution in si_app by deleting data source and clearing data_source_dict

The test can fail in this way by running pip3 install pytest-repeat; python3 -m pytest --count=2 test/test_plugin_mysql_hdfs.py::test_plugin_hdfs_add:

    def test_plugin_hdfs_add():
        si_app.delete_data_soruce(hdfs_ds_dict)
        dict1 = si_app.get_data_source_dict(ds_name=hdfs_ds_dict['ds_name'])
>       assert dict1 is None
E       AssertionError: assert {'_sa_instance_state': <sqlalchemy.orm.state.InstanceState object at 0x7fd7bfbe38b0>, 'created_date': datetime.datetime(2021, 8, 25, 2, 14, 1), 'ds_desc': 'created by unittest of hdfsindex', 'ds_name': 'hdfs1', ...} is None

Such failure occurs while running on test_plugin_mysql_add, and can be fixed in the same way.

It may be better to clean state pollutions so that some other tests won't fail in the future due to the shared state pollution.