Revamp of the rax.py inventory plugin

Eliminate the RAX_REGION environment variable; iterate through all regions available to an account and present servers in region groups; default to using server name to identify servers in groups; set ansible_ssh_host key for each server; utilize the _meta key to provide all the hostvars for each server; all grouping by web, db, sql, lb, app based on server name; pretty print JSON output for some human-readable inventory action

The only special consideration here is that we're no longer referencing instances by IP address -- before, it looked like this:
{'undefined': ['192.168.1.5', '192.168.1.2']}

Now, we reference by the server name:
{'DFW': ['myhost.example.com', otherhost.example.com', 'myserver'] }