add terraform 0.12 support

fix finding asg group name nodes, fixes #8

- autogenerate passwords and cookie secret

- add ability to provide volume type and size
- add ability to provide additional security groups both for ELB and
RabbitMQ nodes
- add create_before_destroy for launch configuration
- consistent resource naming to allow creating multiple clusters
- add min_size max_size and desired_size vars for ASG

Merge pull request #3 from Dmitry1987/bugfix/readme-variable-name-fix

Readme duplicate variable name in config fix

rabitmq terraform cluster