[FAB-12553] add ability to add nodes in Raft tests

Refactors Raft ordering service UT to support reconfiguration,
i.e. addition/removal of new nodes, introducing capability to
inject new chains into mocked network.

Change-Id: Ieb7f8973002ce2c70a59c7985aa09fc9fd0839ed
Signed-off-by: Artem Barger <bartem@il.ibm.com>

Author: C0rWin