Remove radmin credentials from create_heat_client

auth_token is enough for authentication
in creating a heatclient.Client instance.

Currently both username/password and auth_token
are passed as argument in create_heat_client

Fixes: Bug 1251203

Change-Id: I3cce6b35fc40f4ff4cc45b04714067956fb1869d
Merge "Remove radmin credentials from create_heat_client"