core:[RFE] Improve power management proxy select..

[RFE] Improve power management proxy selection

this patch resolves the following scenario:

- cluster-1
  - host-X
  - host-Y
- cluster-2
  - host-Z

- Restart host-Y - succeed
- Restart host-X - Fail: "General command validation failure."

The patch does not consider a host as a proxy candidate if it is in the
Reboot state

Change-Id: I7a8178b0922365a456a6f3b09943f06f9ac6fc2d
Signed-off-by: Eli Mesika <emesika@redhat.com>