heat_integrationtests.scenario.test_server_signal ignores tempest configuration

[Stefan Nica]

The following tempest heat_plugin configuration options are ignored by the heat_integrationtests.scenario.test_server_signal.ServerSignalIntegrationTest test cases:

   - the conf.minimal_instance_type flavor is used in both test cases -
 test_server_signal_userdata_format_raw and test_server_signal_userdata_format_software_config - even though the latter should use conf.instance_type, the flavor associated with the image being used by the test case (conf.image_ref)
   - the default 'public' network hardcoded in the template file is used instead of the value configured in conf.floating_network_name
   - the default 'heat-net' network hardcoded in the template file is used instead of the value configured in conf.fixed_network_name

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/490885