Eliminate grains.cloud_provider (in preference to grains.cloud) from SaltStack

This variable can be entirely derived from grains.cloud, and it
simplifies the configuration somewhat. (Or someone convince me I'm
wrong. I'm happy to be wrong here.)

cluster/vagrant/provision-master.sh

@@ -81,7 +81,6 @@ grains:
   etcd_servers: '$(echo "$MASTER_IP" | sed -e "s/'/''/g")'
   api_servers: '$(echo "$MASTER_IP" | sed -e "s/'/''/g")'
   cloud: vagrant
-  cloud_provider: vagrant
   roles:
     - kubernetes-master
   runtime_config: '$(echo "$RUNTIME_CONFIG" | sed -e "s/'/''/g")'