Allow custom base url

[HuyNguyen7994]

My organization is using teamsite.<tenant>.com instead of normal <tenant>.sharepoint.com. Hence, when I tried to connect it returned following error:

Test failed: Failed to initialize kernel : <class 'robust_session.RobustSessionError'> : Error on attempt #5: HTTPSConnectionPool(host='teamsite.<tenant>.sharepoint.com', port=443): Max retries exceeded with url: /sites/<mysite>/_api/contextinfo (Caused by NewConnectionError('<urllib3.connection.VerifiedHTTPSConnection object at 0x7f219f728320>: Failed to establish a new connection: [Errno -2] Name or service not known',))

I suggest to not automatically append .sharepoint.com at the end of the tenant

dss-plugin-sharepoint-online/python-lib/sharepoint_client.py

Line 136 in f7a9ef9

self.sharepoint_url = tenant + ".sharepoint.com"