cluster: make the cluster config "configurable"

[sgotti]

Now the clusterconfig is fixed to its hardcoded default. To make it consistend above all the members of the cluster (keepers, sentinels, proxies) it should be defined in an etcd key.

If updated all the cluster members should reconfigure themself to use the new values.

[jberkus]

As far as I can tell, there's no cluster configuration at all possible? I don't see a config file, or any documentation for one.

[sgotti]

@jberkus Yeah sorry. It's not there but we are working hard on it. Actually the temporary hack is to write directly to etcd (see for example https://github.com/sorintlab/stolon/blob/master/doc/syncrepl.md) but in the next days we'll remove it and merge a stolonctl to control it and other things (#28) so I'll suggest to wait since this will be a breaking change on the cluster data format.

[sgotti]

An initial implementation of stolonctl and related documentation (#28) with ability to manage cluster config has been merged (#63)