ReplicaSet support

Solves bug #493 

PR includes:

*  /etc/init.d/contextBroker has been fixed to take into account the new parameter BROKER_DATABASE_RPLSET. **Edit: done**
* Install & Admin manual describes the usage of new `-rplSet` CLI option and BROKER_DATABASE_RPLSET parameter. **Edit: done**

The PR is now mergeable.
Fixed in f39803f.