[rally] enable heat plugin
[rally] enable heat plugin

Devstack guys removed heat from their tree[*]. They did not check that it
was used. It broke our gates.

[*] - https://github.com/openstack-dev/devstack/commit/77a7296248b2aae4ce878e33e05575748e7e4131

Change-Id: I52ff22176c7cdbb1440ade28edd0fdc84db002aa

Tesora/tesora-project-config,Tesora/tesora-project-config,openstack-infra/project-config,openstack-infra/project-config