* refactor gpload test file TEST.py

1. migrate gpload test to pytest
2. new function to form config file through yaml package and make it more reasonable
3. add a case to cover gpload update_condition arggument

* migrate gpload and TEST.py to python3.6
new test case 43 to test gpload behavior when column name has capital letters and without data type
change some ans file since psql react different

* change sql to find reuseable external table to make gpload compatible in gp7 and gp6
better TEST.py to write config file with ruamel.yaml moudle
Co-authored-by: NXiaoxiaoHe <hxiaoxiao@vmware.com>