Fix rally plugins relative imports

We are not able to use relative imports for plugins that are
outside of rally package.

To fix this we should add to sys.path all directories with Rally
plugins.

This is quite easy fix. But in any case it includes functional test
that checks that this actually works in real life.

+ Fix of Rally gates to copy whole tree not just *.py files from
  rally-jobs/plugins/

Change-Id: Ie53dc2ba5c32a5e83d84d0ea5703e2e04b10ff33
Closes-bug: #1446923

rally-jobs/plugins/fake_plugin.py

@@ -19,6 +19,9 @@ import time
 
 from rally.benchmark.scenarios import base
 
+# This is used to test relative import
+from test_relative_import import zzz
+
 
 class FakePlugin(base.Scenario):
     """Fake plugin with a scenario."""
@@ -37,5 +40,6 @@ class FakePlugin(base.Scenario):
 
         :param factor: influences the argument value for a time.sleep() call
         """
+        zzz.some_very_important_function()
         self._test1(factor)
         self._test2(factor)

rally-jobs/plugins/test_relative_import/zzz.py

@@ -0,0 +1,5 @@
+# This module is used just for test that relative imports work well
+
+
+def some_very_important_function():
+    return 42

tests/ci/rally-gate.sh

@@ -36,7 +36,7 @@ RALLY_PLUGINS_DIR=~/.rally/plugins
 
 mkdir -p $RALLY_PLUGINS_DIR
 if [ -d $PLUGINS_DIR ]; then
-    cp -r $PLUGINS_DIR/*.py $RALLY_PLUGINS_DIR
+    cp -r $PLUGINS_DIR/ $RALLY_PLUGINS_DIR
 fi
 
 if [ -d $EXTRA_DIR ]; then