core: After enable concurrent option under host...

After enable concurrent option under host power management fencing begin failed

This bug was probably caused by a race:
From the logs its seems that a NULL proxy host is attempt to be used.

There is a check for a valid proxy in canDoAction but not all the
execution paths checks for proxy
Checking for proxy should be also in execute for
single/dual[sequential,concurrent] and the flag that indicates if the
operation succeeded should be set accordingly

Change-Id: I139a7de9a4bb24a56523073b800b929a31fcdc6a
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=977689
Signed-off-by: Eli Mesika <emesika@redhat.com>