Add a runner for contrail-test Juniper tests

- add steps in tcp_tests/templates/opencontrail to prepare tests
- add a fixture and a test runner 'test_opencontrail' that gets
  the two variables from environment to pass them to test runner:

  export OPENCONTRAIL_TAGS=smoke
  export OPENCONTRAIL_FEATURES=neutron

- fix docstrings for fixtures
- add print of the environment variables that can be used in tests
  and current defaults (taking into account the exported variables)
diff --git a/tcp_tests/tests/system/conftest.py b/tcp_tests/tests/system/conftest.py
index afee802..90f0fd4 100644
--- a/tcp_tests/tests/system/conftest.py
+++ b/tcp_tests/tests/system/conftest.py
@@ -20,6 +20,7 @@
 from tcp_tests.fixtures.salt_fixtures import *
 from tcp_tests.fixtures.common_services_fixtures import *
 from tcp_tests.fixtures.openstack_fixtures import *
+from tcp_tests.fixtures.opencontrail_fixtures import *
 
 __all__ = sorted([  # sort for documentation
     # common_fixtures
@@ -42,4 +43,6 @@
     # openstack_fixtures
     'openstack_actions',
     'openstack_deployed',
+    # component fixtures
+    'opencontrail',
 ])