Add a new module which could export test cases from Polarion document

[zhaohuijuan]

Add a new module polarion_tc which could export test cases from Polarion documents.

Export test cases of documents in a space to json or text file like below, the exported json file could be used as database for os-tests generating Polarion log file.
{
"work_item_id": "VIRT-296346",
"title": "os_tests.tests.test_cloud_init.TestCloudInit.test_check_cloudinit_cfg_no_wheel",
"documents": [
"OS-TESTS TestCloudInit Test Cases"
]
}

Usage examples:

1. Export test cases of all documents in a space to tc.json (overwrite)
   python polarion_tc.py --project RHELVIRT --space "OS-Tests" --out polarion_tc.json --json
   or if installed tipset:
   polarion_tc --project RHELVIRT --space "OS-Tests" --out polarion_tc.json --json

2. Export test cases of a single document and append/merge into existing tc.json
   python polarion_tc.py --project RHELVIRT --space "OS-Tests" --document "OS-TESTS TestCloudInit Test Cases" --out polarion_tc.json --json --append
   or if installed tipset:
   polarion_tc --project RHELVIRT --space "OS-Tests" --document "OS-TESTS TestCloudInit Test Cases" --out polarion_tc.json --json --append

3. Print to stdout (text)
   python polarion_tc.py --project RHELVIRT --space "OS-Tests"
   or if installed tipset:
   polarion_tc --project RHELVIRT --space "OS-Tests"

[zhaohuijuan]

This file is added to os-tests repo, closing this MR
FYI - virt-s1/os-tests#633