core: adding API for Cluster Fencing Policy

This patch adds support for get/set/add cluster fencing policy from the
REST API

clusters retrieved from REST will have now the following section
(example):

<fencing_policy>
        <enabled>true</enabled>
        <skip_if_sd_active>
            <enabled>false</enabled>
        </skip_if_sd_active>
        <skip_if_connectivity_broken>
            <enabled>false</enabled>
            <threshold>50</threshold>
        </skip_if_connectivity_broken>
 </fencing_policy>

Change-Id: I086e70890096b5e25cfeb8d519f235952087aa4f
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1127592
Signed-off-by: Eli Mesika <emesika@redhat.com>