Bugfix: known_hosts file was not always deleted when the cluster was terminated.
Update documentation related to the automatically created known_hosts
file when using `elasticluster ssh` and `elasticluster sftp`
Add option `ssh_pipelining` to enable/disable ssh pipelining when running ansible. By default it's enabled.
Add fix to allow loading of a cluster created *before* the addition of option `ssh_pipelining`