core: skip failing proxy when waiting for status

This patch fix a bug when we have a preferred proxy (in 6the same
cluster as the fenced host) that failed to perform the desired
operation.

In this case, we should remember the failed proxy inside the "get
status" loop and use the one that had succeeded (if exists)

Change-Id: I2b846444ff795e7099b61f6d0be8909402b7bfa1
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1141514
Signed-off-by: Eli Mesika <emesika@redhat.com>