Crash on int env vars

[guillem21b]

In Vault we are allowed to put integers as env vars, but to registering the task definition we must use strings. Since there isn't a cast to str, the deployer crashes:

INFO:root:Registering task definitions
CRITICAL:root:Parameter validation failed:
Invalid type for parameter containerDefinitions[0].environment[5].value, value: 999999, type: <class 'int'>, valid types: <class 'str'>
Traceback (most recent call last):
  File "ecs_deployer/run.py", line 55, in <module>
    main(procfile_path=args.procfile, ecr_image=args.image)
  File "ecs_deployer/run.py", line 26, in main
    ecr_image)
  File "/app/ecs_deployer/services.py", line 57, in register_task_definitions
    family)
  File "/app/ecs_deployer/aws.py", line 15, in register_task_definition
    **task_definition)
  File "/usr/local/lib/python3.6/site-packages/botocore/client.py", line 357, in _api_call
    return self._make_api_call(operation_name, kwargs)
  File "/usr/local/lib/python3.6/site-packages/botocore/client.py", line 634, in _make_api_call
    api_params, operation_model, context=request_context)
  File "/usr/local/lib/python3.6/site-packages/botocore/client.py", line 682, in _convert_to_request_dict
    api_params, operation_model)
  File "/usr/local/lib/python3.6/site-packages/botocore/validate.py", line 297, in serialize_to_request
    raise ParamValidationError(report=report.generate_report())
botocore.exceptions.ParamValidationError: Parameter validation failed:
Invalid type for parameter containerDefinitions[0].environment[5].value, value: 999999, type: <class 'int'>, valid types: <class 'str'>

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "ecs_deployer/run.py", line 58, in <module>
    raise Exception(e)
Exception: Parameter validation failed:
Invalid type for parameter containerDefinitions[0].environment[5].value, value: 999999, type: <class 'int'>, valid types: <class 'str'>
Exited with code 1