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
Ok, let's keep as it is now.

Thus, NTC.