Updates to gen.py
Updates to gen.py

- Allow mapping from service name to filename
  (e.g., lambda => awslambda)
- Changes for bulk creation:
  - The code generator can output to stdout via:
    python3 scripts/gen.py ses
  - Using the -d flag will write into file(s) in that directory
    python3 scripts/gen.py -d troposphere ses sqs

cloudtools/troposphere,cloudtools/troposphere