[SPARK-5668] Display region in spark_ec2.py get_existing_cluster()

Show the region for the different messages displayed by get_existing_cluster(): The search, found and error messages.

Author: Miguel Peralvo <miguel.peralvo@gmail.com>

Closes #4457 from MiguelPeralvo/patch-2 and squashes the following commits:

a5514c8 [Miguel Peralvo] Update spark_ec2.py
0a837b0 [Miguel Peralvo] Update spark_ec2.py
3923f36 [Miguel Peralvo] Update spark_ec2.py
4ecd9f9 [Miguel Peralvo] [SPARK-5668] Display region in spark_ec2.py get_existing_cluster()