Fix specification of elasticbeanstalk tier

According to the API: http://docs.aws.amazon.com/elasticbeanstalk/latest/api/API_CreateEnvironment.html
The ``member`` part of the Tier key names is not valid.
I was always getting:
{"Error":{"Code":"InvalidParameterValue","Message":"Environment tier definition not found","Type":"Sender"},"RequestId":"xxxxx"}

This allows an environment to be created and to specify the tier_name and tier_type.
Merge branch 'beanstalk_tier' of https://github.com/stuart-warren/boto into beanstalk_tier