Added new frontend to vary the user submission rate during the experiment.

Changed the start frontend script so that the ec2_exp script
can fill in the frontend class name.
Fixed bug with round robin scheduler.

Previously, when the round robin task scheduler tried to launch a task for a
given user and all tasks in that job had already been launched, the user lost
that particular spot. This commit changes that behavior so that instead, if
all tasks in the particular job were launched, the scheduler tries to launch
another task for the same user.