Added --ignore-test-failures CLI flag

Tox venv commands are expected to return a code that indicates
test cases final outcome:

 - 0 => any test vase has failed
 - 1 => some test case has failed
 - not in [0, 1] => any other error

This change handles the special case when some test cases have
failed. In any of below cases:
 - JENKINS_URL environment variable is defined on controller node
 - --ignore-test-failure flag is passed to IR plugin
then the plugin runs without treating it as an error.

This has been introduced because when running the plugin from
a Jenkins slave host, Jenkins itself is expected to check test
case have failed looking at produced Junit XML artifacts.

Change-Id: Ia7d7f736ae18a7561dced2271228d86ca658e892

tools/run_tests.py

@@ -53,6 +53,7 @@ def main():
         succeeded = run_tests()
         if succeeded:
             LOG.info('SUCCEEDED')
+            sys.exit(0)
         else:
             LOG.info('FAILED')
             sys.exit(1)