Please review ticket 47590 (take #3): add/split functions around replication

[thierry bordaz]

This review takes into account the recommendations of previous review:

* Create a Agreement class in brooker
  * mv init/status/schedule/create in that new class
  * mv createDefaultReplMgr into the brooker replica class with the
    function create_repl_manager()
  * Handling of error condition with exceptions

What is not implemented in that review, that will be implemented with 
https://fedorahosted.org/389/ticket/47600:

* Cleanup of createAgreement (use of exception). This function will
    likely be replace by agreement.create().
  * Cleanup of enableReplication (use of exception, move under
    replica.create())

https://fedorahosted.org/389/attachment/ticket/47590/0003-Ticket-47590-CI-te...