This introduces a new setting OPENSTACK_USE_SIMPLE_TENANT_USAGE
as a replace of SimpleTenantUsage in OPENSTACK_NOVA_EXTENSIONS_BLACKLIST.
We received feedbacks on the need to control whether SimpleTenantUsage
is used or not, so we need a new setting before retiring
OPENSTACK_NOVA_EXTENSIONS_BLACKLIST.
Change-Id: Ib786ea2ac232fcd851f4b276b9144c14c0b62a0a