EC2 configurable workers

Added option to configure number of worker instances and to set SPARK_MASTER_OPTS

Depends on: https://github.com/mesos/spark-ec2/pull/46

Author: Allan Douglas R. de Oliveira <allan@chaordicsystems.com>

Closes #612 from douglaz/ec2_configurable_workers and squashes the following commits:

d6c5d65 [Allan Douglas R. de Oliveira] Added master opts parameter
6c34671 [Allan Douglas R. de Oliveira] Use number of worker instances as string on template
ba528b9 [Allan Douglas R. de Oliveira] Added SPARK_WORKER_INSTANCES parameter
(cherry picked from commit 4669a84ab10296e01a5fbbae1de9574b793b7ed5)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>