Rackspace inventory: add support for new environment variable RAX_PRIVATE_IP

Add support for new environment variable, RAX_PRIVATE_IP which will the ansible_ssh_host value to the private IP of the instances.  Additionally removes redundant parenthesis and is a redo of the original documentation.
```
RAX_PRIVATE_IP=yes rax.py --list
```

Additionally, some changes occurred at Rackspace where new customers may not have all endpoints for servers in all data centers, despite pyrax reporting that they do.  This is part of a larger issue that is resolved by the second commit, bd323d0