Description of problem:
Power Management failed if no pm_proxy preferences is defined for the host.
The pm_proxy_preferences is a per-host field in vds_static that defined how to look for a proxy host. (cluster then DC or vice versa)
In case that pm_proxy preference is not defined , it should be taken from the FenceProxyDefaultPreferences configuration value which is defaulted to "cluster,dc"
However, the code that checks for existance of pm_proxy preferences checks for null only and if it is an empty string , it is used 
The error on the log is :

 2013-06-02 13:04:25,013 ERROR [org.ovirt.engine.core.bll.FenceExecutor]    
 Illegal value in PM Proxy Preferences string , skipped

Version-Release number of selected component (if applicable):


How reproducible:
Always

Steps to Reproduce:
1.Configure a Host with PM , so not assign value to the Source field
2.Try to fence this Host
3.

Actual results:
Error 
2013-06-02 13:04:25,013 ERROR [org.ovirt.engine.core.bll.FenceExecutor]    
 Illegal value in PM Proxy Preferences string , skipped


Expected results:
Host should use default "clsuter,dc" value from the config and Host shouyld be fenced 


Additional info: