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.

Only inventory "running" EC2 instances. "stopped" is also a valid state and these should not be inventoried.

without a keypair, so this takes care of that scenario

This fixes #1265.