tag ec2 instances with cluster and node name (e.g., aws-slurm-compute003)

This branch adds Name tags to EC2 instances named "clustername-nodename" - it's much easier to identify which instances are which, and belong to which cluster.

The change itself is fairly straightforward: one line in the ec2_boto provider to set the tag, one line in Node.start() to prepend the cluster name. The latter also affects other providers, but it seems the disambiguation would be useful there, too.

I also changed the (internal) Node API, since it seemed better to pass the cluster instance into its nodes, since they use a number of the cluster's members. Let me know if you disagree and I pass the cluster name separately to Node.