rax.py inventory script improvements

This pull adds a number of requested features:

 * Support other OpenStack clouds via `~/.pyrax.cfg` and the `RAX_ENV` environment variable
 * Add flavor based groups (instance size) (eg flavor-4 or flavor-performance2-15)
 * Add image based groups (eg image-2ab974de-9fe5-4f5b-9d58-766a59f3de61 or image-debian-7-wheezy)
 * Add instance UUID based groups (eg instance-eba77620-be18-4abd-b19d-f67c054a04f0)
 * Add meta key_value groups (eg meta_foo_bar) (this is configurable to "match" ec2.py by setting `RAX_META_PREFIX=tag`)
 * Support for secondary groups in addition to a primary group.  When creating a server, use a comma separated list of groups like the following when creating a server using the rax module:
```
    meta:
        groups: foo,bar,baz
```
 * When 'region' is set in an environment in `~/.pyrax.cfg`, only that region is inspected
 * Support for keyring storage of password/api_key is achievable via ~/.pyrax.cfg by utilizing the `keyring_username` configuration

No existing functionality was changed.  This maintains backwards compatibility, just adds the above new functionality.