Description of problem:
fence_rhevm fence_agent group definition missing from the cluster.conf Relax-NG schema.

Version-Release number of selected component (if applicable):
cman/cluster-3.0.12-23.el6_0.4

How reproducible:
Always.

Steps to Reproduce:
1. Add a fence_rhevm fencedevice element to a RHEL6 cluster.conf.  Example:

        <fencedevices>
                <fencedevice agent="fence_rhevm" ipaddr="##.##.###.###" login="USERNAME" name="HOSTNAME" passwd="PASSWORD" power_wait="30" shell_timeout="12" ssl="1"/>
        </fencedevices>

2. Run 'ccs_config_validate'
  
Actual results:
[root@rhel60node-01 ~]# ccs_config_validate 
Relax-NG validity error : Extra element fencedevices in interleave
tempfile:119: element fencedevices: Relax-NG validity error : Element cluster failed to validate content
Configuration fails to validate


Expected results:
[root@rhel60node-01 ~]# ccs_config_validate 
Configuration validates


Additional info:
The identical bock works fine on RHEL5 with system-config-cluster-1.0.57-7
Patch forthcoming.