Run sequentially tempest_heat

Run sequentially tempest_heat

It's forced by design else it remains a few resources.
It also sets fixed_subnet_name and network_for_ssh to avoid falsy
discovery as it seems [1]

[1] http://artifacts.opnfv.org/functest/X5GDYF3B0R2B/functest-opnfv-functest-smoke-jerma-tempest_heat-run-18/tempest_heat/tempest-report.html

Change-Id: Ieb584ddc1c0b4ebb543e9d69277a20aaee0fbac0
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>
(cherry picked from commit c2e3d911d9eaf987d18b3b895778c411e62efc4a)