Add dependency to execo

Execo is required by the python helper scripts to fetch the g5k
environment description.

kameleon/README.md

@@ -25,6 +25,7 @@ frontend$ oarsub "sleep 2h" -l nodes=1,walltime=2
 Once the job is running, connect to your node and install the required components:
 
 ```
+node$ pip2 install --user execo
 node$ gem install --user-install kameleon-builder
 ```