ReplicaSet support

Solves bug #493 

**NOT MERGE** before:

*  /etc/init.d/contextBroker has been fixed to take into account the new parameter BROKER_DATABASE_RPLSET
* Install & Admin manual describes the usage of new `-rplSet` CLI option and BROKER_DATABASE_RPLSET parameter
@kzangeli , what do you think about this? Do you think is a better way of implemeting this? I'm not very confident with the options that I've thought about (a macro or a function passing the connection function as argument).