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.

Fixes #2064.

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

Since callbacks are called with different argument-types, we have to be careful. We support two different distinct cases:

 - The error information can be in one ore more of the following items (msg, stderr or stdout)
 - The res/msg value returned can be a string or a list

This is useful mostly for playbooks that run unattended and for a limited set of systems. In case of provisioninging this plugin (together with a final mail action) helps to get notified when something went wrong, or when the installation finished successfully.

Unfortunately, there is no way to enable/disable a plugin from a playbook. So installing the plugin means all other use-cases (provisioning, troubleshooting, reporting or management) all send mails on failure. Something we may want to fix in the future...

This fixes #1265.