Added required value for openstack_provider for rackspace example

If `openstack_provider:rackspace` is omitted in the configuration, the example
won't work at all against rackspace. Not sure what the actual documentation for
this configuration value should be, but this was a major source of irritation
when I couldn't get the default example working against Rackspace.
Merge pull request #1260 from lflux/website-openstack-rackspace-default

website: add required value for openstack_provider for rackspace example