feat(provision-rackspace): added optional parameter to specify environment

The Rackspace provisioning script has the supernova environment "production" hard-coded in. These changes allow you to specify which environment to use.
I thought about this but held back as I thought it might be more common to do `./provision-rackspace-cluster.sh keypair environment` than to specify a flavour and then the environment. On second thought, backward compatibility is a good idea. I'll switch these around and update the PR.