added clean_tmp for a reservation

py ec2_cmd.py clean_tmp --reservation r-79240412
cleans out everything in /tmp in the reservation, using (prints what it's doing)
ssh -i ~/.ec2/keys/mrjenkins_test.pem ubuntu@10.5.25.46 'sudo rm -rf /tmp/*; df'
Merge branch 'master' of https://github.com/0xdata/h2o