Refactor heat gate testing contrib support

The existing code is running tests twice, one for heat, one for
contrib via adding two commands in tox.

This breaked tox single run. For example:

  tox -e py27 -- heat.tests.test_software_config

Except heat.tests.test_software_config is executed, all test cases in
contrib will be executed also.

The patch added contrib in discover list, which can make sure both
contrib in heat gate testing and does not break single test run.

Change-Id: I014d7b95c64287e1fd04a0ba69fce0de554a9a08
Merge "Refactor heat gate testing contrib support"