Fix wrong inheritance for class HeatBaseTest

Sometimes the Sahara Platform test fails without any messages.
This bug occurs when we run Heat and Sahara Platform tests together.
If we run the Sahara test only, everything will be OK. The problem is
in wrong inheritance for class HeatBaseTest in heatmanager.py.

Change-Id: I88c4100d3603cffc4f353386490802fc6f6953b7
Closes-Bug: #1367743