Fix doc for usage of python clients in scenario tests

Scenario tests has been migrated from official python clients to
Tempest clients.
Documents for the same needs to be fixed.

This patch fix the README & HACKING file for above changes.

This patch also extends hacking rule of not import python clients
for scenario tests too.

Change-Id: Ieb19a2c0b09f00fb3d4f6c7c73541275a4cf24ae
Merge "Fix doc for usage of python clients in scenario tests"