set connection state ~atomically, preventing race condition in setup threads
Improve message error when trying to load a cluster saved with a previous version of elasticluster.

Partially fixes issue #126
Merge pull request #125 from jwm/ec2_vpc_atomic_connect

Set EC2 and VPC handles atomically
Prevent creation of a node with an already existing name.
Fixes issue #127
Add a configuration option `thread_pool_max_size` to limit the maximum amount of processes that are created when starting virtual machines.
Add `migrate` command, to update cluster classes saved to disk from older versions of elasticluster.
Do not terminate instances just because we cannot connect to them.
Possibly fixes issue #128
Update `migrate` command: add `thread_pool_max_size` attribute.
Fix nasty bug in migrate command
Improve migration of `Cluster.known_hosts_file` attribute.