Make soap endpoint configurable and remove legacy flag

[ipetresf]

W-5497231: Phyton - Auth, Rest and SOAP endpoints should be made configurable W-5500244: Python - Remove the legacyToken query string parameter support from all SDKs W-5500256: Python - Cache the SOAP endpoint for 15 minutes

Work done:

• Made SOAP endpoint configurable
• Add a default value for soap_endpoint when call to /endpoints/soap fails
• Add caching for /endpoints/soap call
• Remove legacy flag
• Update SOAP header to send tag
• Update README.md