Force pseudo-tty allocation in spark-ec2 script.

ssh commands need the -t argument repeated twice if there is no local
tty, e.g. if the process running spark-ec2 uses nohup and the parent
process exits.

Without this change, if you run the script this way (e.g. using nohup from a cron job), it will fail setting up the nodes because some of the ssh commands complain about missing ttys and then fail.

(This version is for the master branch. I've filed a separate request for the 0.8 since changes to the script caused the patches to be different.)