Added Rackspace provider-specific configuration option: metadata

Adding Rackspace provider-specific configuration option: metadata

Rackspace API documentation here:
http://docs.rackspace.com/servers/api/v2/cs-devguide/content/MetadataSection.html

This was borrowed from the OpenStack plugin:
https://github.com/cloudbau/vagrant-openstack-plugin

p.s. My first pull request, so feel free to bash/teach me the right way to do things