Python ALTS example code missing for client and server authorization

[chalin]

Recent changes (#527, #539) have brought to light that Python example code is missing for client and server ALTS authorization:

• See https://grpc.io/docs/languages/python/alts/
• Contrast this with, for example, https://grpc.io/docs/languages/java/alts/

cc @jiangtaoli2016 @gnossen @srini100