core: fix module name in fencing logs(#652581)

This patch changes the term "Fencing" to "Fence" in all log keys &
messages and class names.

All files marked as the code was deleted & added are a result of a class
name change while the class implementation code is completely identical.

Change-Id: Ica3ced588e67943038016079d0aa51f19d3dfea0
Signed-off-by: Eli Mesika <emesika@redhat.com>