Enable Cluster wide deployment of Marketplace packages

We need a way to push install/update of set of MP on several nodes of a cluster.

For that, the first step is to have a common target configuration.
With the current infrastructure, the solution may be to have a dedicated "cluster channel".

• a way to share some kind of "packages repository" that is cluster specific
• add a new Package Channel that will be dedicated to Cluster (simple http repository for now)
• a way to trigger the installation on all nodes : one after an other
• add a new NuxeoCtl command : `nuxeoCtl clusterUpdate` that automatically fetch and install Cluster packages