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.
let's append `[environment]` at the end of the argument list as $3. This will save others from having to fix up their scripts (e.g. if someone is on 0.14 and uses `./provision-rackspace-cluster.sh keypair flavor`, this change would mean that `flavor` would be parsed as the environment).