Don't use all hosts as potential master hosts

[widhalmt]

Right now we assume every host is a potential master and use the whole list of hosts for cluster building. If the cluster is expanded in a later stage every new node will be used as master, too.

We need the following:

• Option to set node.master to decide which nodes are potential masters and which are not
• Define the list of initial masters during initial setup, write the list into a (fact?)file on the CA host and stick to that in an already deployed cluster
• Some extra logic to check for violations of best practices (odd number of potential masters, not using master only nodes for communication etc)