add cache_subnet_group_name param, cache_security_group can be None

This PR adds the ability to set the cache_subnet-group_name that is required when deploying into a VPC. The current code sets it to 'default' which is probably erroneous for most people.

This also removes the default value for the cache_security_groups param (which is the value 'default') as it needs to be blank when deploying into a VPC.