Renamed on_no_hosts_matched/on_no_hosts_remaining to correct names in noop callback, added methods to callback plugin examples.

This plugin is not quite ready as it needs to support --host <hostname> and --list, like the other inventory plugins, but we should get this tweaked and include it once it is standardized.  Watch this space!

This is particularly important if you've manually ensured you have write
access via group permissions and not by owner.

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.