Move regions until after authentication; otherwise pyrax.regions is an empty tuple and no inventory is returned.

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

Add set_setting to alleviate requirement that ~/.pyrax.cfg exists

Add regions as a optional env var to narrow our inventory scope

Clean up

The rax inventory plugin provides a way to discovery inventory in the
Rackspace Public Cloud by way of pyrax, the official SDK. Grouping will
be done if a group:name is found in the instance metadata. When a single
host is queried all the instance details are returned with a rax_
prefix.

Because inventory plugins cannot take extra arguments, ENV variables
must be set to point to the pyrax compatible credentials file and the
region to query against.