Drop direct engine support
Drop direct engine support

This change remove support of direct engine in Sahara.
Unit tests related to direct engine were removed.

Implements blueprint: remove-direct-engine

Change-Id: Iba292996dd052a069a98f1024441255a15c0f734

openstack/sahara-scenario,openstack/sahara-tests,openstack/sahara-tests,esikachev/sahara-tests,openstack/sahara-tests,openstack/sahara-tests,esikachev/sahara-tests,openstack/sahara-scenario,openstack/sahara-scenario,esikachev/sahara-tests