Try to silent virtualenv creating log

Change-Id: I1ed24fc0a9e49732e2004942ab485e5f91d14563
diff --git a/src/com/mirantis/mk/Python.groovy b/src/com/mirantis/mk/Python.groovy
index 1844d42..6533276 100644
--- a/src/com/mirantis/mk/Python.groovy
+++ b/src/com/mirantis/mk/Python.groovy
@@ -33,7 +33,7 @@
  * @param cmd    Command to be executed
  */
 def runVirtualenvCommand(path, cmd) {
-    virtualenv_cmd = ". ${path}/bin/activate; ${cmd}"
+    virtualenv_cmd = ". ${path}/bin/activate > /dev/null; ${cmd}"
     echo("[Python ${path}] Run command ${cmd}")
     output = sh(
         returnStdout: true,