core: Enable to modify/remove PM agents via API.

After adding two power management agents to a host, impossible to
remove/override second agent

This patch enables to remove/modify PM agents via API.
The problem was that the Host entity given to the API mapper contained
the former settings and nothing cleans it in order to set the actual new
settings properly
This patch fixes the following flows

1) Remove PM agents
2) Add a Secondary PM agent when Primary PM agent is defined
3) Remove a Secondary PM agent
4) Remove Primary PM agent and turning the Secondary PM agent to be
primary by changing the order from 2 to 1
5) remove/modify with the backward-compatibility syntax directly from
   the power management node (a primary PM) without sending the agents
   node

Change-Id: I8a1025ccb4a50f5a5d45746b44b93c12fb14a268
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1099903
Signed-off-by: Eli Mesika <emesika@redhat.com>