Use python3 for the mininet entry point

Change-Id: Idfaff80c3638c8aad7a8affa40ae480aaf9d40ca
diff --git a/.gitignore b/.gitignore
index c96efd1..8673b67 100644
--- a/.gitignore
+++ b/.gitignore
@@ -10,3 +10,6 @@
 
 # ignore dependent chart dirs
 charts
+
+# IntelliJ
+.idea